Happened to me last night, and I was pissed.
 
After getting 6pool rushed into banelings, he said GG after he killed my marines.  I immediately left without saying anything.  It just seems like you are kicking dirt in the face of the loser when they are on the ground. "Oh. good game! hahah!"
 
It probably didn't help because I just got out of a 45 minute match from hell.  How the hell do you counter mass mutalisks as terran?!  I made thors, but they they tore him up but they are so god damn slow.  He just kept hitting other parts of my base from where my army was.  He was able to hit my expansions and kill all my SCVs before my thors even made it have way.  I tried some vikings, but I'd have to make so many to be able to counter him.  All it would take after that is some zerglings to fuck me over if I Massed vikings.

When anyone masses up anything, at that point a few counter units won't do much at all, just because he'll be throwing out so much damage that it doesn't matter, what I'd suggest is more scouting,if a zerg player throws down a spire, a lot of the time, they'll get a bunch of mutas, it's not like the terran where you'll only throw down a starport sometimes for a few vikings and medvacs, so scouting and taking a look at what your opponents army is compromised of is a very good idea. If you are getting contained in your base, the first thing you want to do is break the containment to give yourself some room to move, tactics like madvac drops are good for this, or perhaps cloaked ghosts, all you want to do for the moment is get pressure off your base, so try doing some sneaky counters at that point.
 
I don't like the idea of gg'ing before the loser, but i have done it once or twice myself, only when I've beaten them and they just fly their base off, or hide an scv in the corner of the map just to be silly, in a legit game, I wouldn't say it first, does seem kind of egotistical.

Make some missile turrets. They're pretty ridiculous right now. 
 
Just place a few in a position where they can hit mutalisks if they try to attack your mineral line. Shove the SCVs that he's focus firing into the Command Centre and repair the turrets if he decides to attack those. If you have another thor or two on top of that, it basically stops the mutalisks dead cold.
 
If you do decide to mass vikings, don't sit them in your base. Go kill off overlords when there's no fights going on. Force him to make hydralisks, or some other anti-air to stop his food from going into the red.
 
As for offensive GG'ing, it's terribly bm.

" Happened to me last night, and I was pissed.  After getting 6pool rushed into banelings, he said GG after he killed my marines.  I immediately left without saying anything.  It just seems like you are kicking dirt in the face of the loser when they are on the ground. "Oh. good game! hahah!""

Yeah, I'm not a fan of someone GG'ing as soon as they feel that they've won. I refuse to quit at that point. I'll immediately just start scattering all of my buildings and start building as many command centers as I can.

I don't think it's such a big deal who says "gg" first;  just telling someone "good game" is supposed to be good sportsmanship.  Since you have to go out of your way to do it, instead of just ignoring the opponent (which isn't bad sportsmanship, by the way), he's probably not being a major dick unless he says something else.

Whoa. So you lost and instead of the winner saying rofl noob! He said GG and you're butt hurt over that? Christ...

I GG at the looser as a tell that I'm coming for him with my huge army. :P

" Whoa. So you lost and instead of the winner saying rofl noob! He said GG and you're butt hurt over that? Christ... "

Them GG'ing before I do If I'm losing really annoys me too. But I've lost so many games in a row today that gold league can go suck a dick for all I care. 
 
So maybe I'm not in the most stable mood at the moment...

" Happened to me last night, and I was pissed.  After getting 6pool rushed into banelings, he said GG after he killed my marines.  I immediately left without saying anything.  It just seems like you are kicking dirt in the face of the loser when they are on the ground. "Oh. good game! hahah!"  It probably didn't help because I just got out of a 45 minute match from hell.  How the hell do you counter mass mutalisks as terran?!  I made thors, but they they tore him up but they are so god damn slow.  He just kept hitting other parts of my base from where my army was.  He was able to hit my expansions and kill all my SCVs before my thors even made it have way.  I tried some vikings, but I'd have to make so many to be able to counter him.  All it would take after that is some zerglings to fuck me over if I Massed vikings. "

You need to play it like the old bw mass muta builds. If he's making that many mutas he probably wont have gas for anything else, so he'll be doing nothing but expanding to secure more gas, and making drones/lings. What you need to do is just mass thors and either marines or hellions to take care of the lings. If you can afford the gas to add more factories then definitely get some hellions with preigniter, but you'll probably be low on gas, so just drop as many rax as you can and start macroing.

You have to realize that his entire strategy will be abusing the immobility of thors. He's just going to poke around at all of your bases to see if there are any weak spots to harass, and then when you eventually move out he will probably just counter your main and try to take out your production. You need to make lots and lots of turrets at all the important parts of your base, i.e. command centers and factories to protect against his backstab, or at the very least slow him down quite a bit and damage his mutas in the process, and then just go for the jugular.

Also, never making vikings to counter mutas. Ever. And you're right, offensive GGing is the biggest "fuck you" there is in starcraft.

" @Cataphract1014 said:

" Happened to me last night, and I was pissed.  After getting 6pool rushed into banelings, he said GG after he killed my marines.  I immediately left without saying anything.  It just seems like you are kicking dirt in the face of the loser when they are on the ground. "Oh. good game! hahah!""

This just screams "sore loser" to me.  I man, I could understand if he said "lol gg noob faggot i raped u rofl gb2 halo" or something, but seriously? :/ "

" Whoa. So you lost and instead of the winner saying rofl noob! He said GG and you're butt hurt over that? Christ... "

You have no idea what you are talking about. They aren't saying GG to be polite, they are saying GG to tell you, "Get the fuck out of the game, it's over". It's a dick move because they are implying that the game is over before the other person decides that it is. If someone does this you have every right to go float buildings around the map or hide pylons. 

@Semition said:

" Make some missile turrets. They're pretty ridiculous right now.   Just place a few in a position where they can hit mutalisks if they try to attack your mineral line. Shove the SCVs that he's focus firing into the Command Centre and repair the turrets if he decides to attack those. If you have another thor or two on top of that, it basically stops the mutalisks dead cold.  If you do decide to mass vikings, don't sit them in your base. Go kill off overlords when there's no fights going on. Force him to make hydralisks, or some other anti-air to stop his food from going into the red.  As for offensive GG'ing, it's terribly bm. "

And never make vikings vs mutalisks. However if you do have vikings vs mutalisks, you need to use them by sitting them atop whatever other anti-air you have and abusing their range to tack on additional damage if the zerg tries to poke in and snipe something. Mutas' move speed is like 3.75 or something while vikings is 2.75, if you are out killing ovies with your vikings the mutalisks will chase you down while picking off just about every one of your vikings.

How the hell do you counter mass mutalisks as terran?!

 2-3 ravens (seeker missile) and vikings (range and firepower) are your best bet against mass muta for me, lure them in with the vikings range, if they rush you, seeker missile.  Have this with a combo of bioball + medivacs and it shouldnt be hard to start pressing map control against macro zerg. "

I equate saying GG as the winner before the person admits defeat like a sports team that is crushing the other team taking out their starters for the backups.  It is insulting to the other team.
 
Sure you can say, "We'll you shouldn't suck so much."  But if the player is better than you, or if you just make a mistake you should be the one to say, "Yeah, I've lost."
 
Losing doesn't upset me too much, although getting 6pooled is super gay, but the winner basically rubbing it your face is what annoys me.  It's like spawn camping in a shooter.  It is bad manners.
 
Anyway, more to the point of the mass mutalisks.  We had both mined out our mains and naturals when we started to go for the other expansions.  I hit his at gold with a quick marine drop and ran like a little girl, and we traded back and forth.  And honestly what I should have done at the end was just sit my whole army on my last expansion and just defend it.  I had some turrets up in my base, but not enough to hold off the 30 mutas he had at any given time.  He tried to get zerglings at one point and I made a ton of hellions with the blue flame upgrade and 3 weapon upgrades and absolutely decimated them.   I pushed into his main after that and killed nearly everything, but once again he pulled mutas out of no where after I killed 2 spires, and finished off my army.
 
He seemed to have a spire at every expo

Turrets make mutas useless. 
 
Then just go do a few thor drops and wreck his economy.
 
And perhaps even Battlecruisers? I don't know how they fair vs. mutas.

GGing as the winner before the loser is bad form and generally classless. I don't think I'd agree with the sports analogy because in sports there is a significant risk of injury, so resting the starters near the end of a blowout is just smart, not bad sportmanship - but that's beside the point.  
 
As for the mass muta question, the best counter for mass anything is to scout enough that you know what he's amassing so you can build up an effective counter force before he actually has a mass force. This is especially important against Zerg, as you'll never be able to outrace them. If you're not prepared for their masses and try to build a counter as their attacking you, it's already too late. 
 
The other strategy is to keep harassing them to prevent them from being able to build up a big force. However, since you play Terran and probably prefer to play more defensive, your best strategy is to scout as much as possible, build an effective counter for what they're doing, wait for them to attack, wipe out their force, then swoop in with your own. 
 
I haven't really dug into SC2 multi yet but if it's anything like SC1, towers chew up Mutas. 
 
But again, scout as much as possible. Knowledge is power.

vikings are how you do anti air as terran

This makes me want to get the game and then gging randomly while playing.

This is making me picture this sort of thing playing out at an RTS LAN party or something.  That'd be hilarious.

I had this happen to me once when a guy rolled into my base with a decent MM force.  He GGed me after taking out my measly few Zealots and Stalkers but must not have noticed the swoosh swoosh of DTs cutting through his forces.  He took out two Pylons and a Gateway before his last guy was put down.  Of course at this point my Warp Prism was spinning behind his mineral line and I had Chronoboosted my other three Gateways down to warp in another set of DTs.  As I began to gleefully slice through his SCVs I said, "GG :D" and he ragequit.
 
It was the best beta ever.

Generally speaking, as other people have mentioned, any time a player masses just one of any unit it can be pretty easily countered. In the case of mass Mutas, making Turrets to cover your base is the best idea for defense, along with perhaps leaving a Thor or two back for defense. You'll want to have the Turrets overlap each other in range a little bit, so even if they snipe off one they'll be taking additional hits plus they won't have free reign in that area of your base while you try to lumber back there with your army.
 
The other thing is that Thor is indeed the correct unit to be producing against Mutalisks. While they are a counter though, be wary of the mindset of them being a "hard counter", as enough Mutalisks with still kill your Thors even with their superior anti-air splash attack. You need to have a bunch of Thors, along with an ample amount of Marines supporting them, and you should have no issues with Mutalisks. Naked Thors (aka Thors by themselves) are far too vulnerable vs Mutalisks by themselves.
 
That's really just a general idea though, and without more information about what was happening during the game it's hard to say what you could've or should've done.

It's rude and obnoxious. However, I do it when I've CLEARLY, CLEARLY beaten someone and they're being really really obnoxious and not leaving (I'm talking making me hunt down flying buildings and things like that), or if they tried something lame and failed. 

One time i was Dueling a guy in wow and it was very close and i lost so i typed GD for God Damnit, but the guy thought i meant "Good duel".

What the hell, I'll post the replay, but beware it is a long one! Over an hour =\
 
It was a good game, but I was still pissed off and I took it out on my poor trash can.
 
Replay!

" What the hell, I'll post the replay, but beware it is a long one! Over an hour =\
 
It was a good game, but I was still pissed off and I took it out on my poor trash can.
 
Replay! "

Agreed this is a pretty crappy thing to do, except when they pull the preemptive gg and then you come back and win. That's pretty much the best thing ever.

Haha, that's pretty messed up. But funny.
 
Also, everyone telling him that he shouldn't be angry or whatever, has no idea what the hell they're talking about.

This happened to me yesterday.  I thought my army of zealots and stalkers would run show, but his dozen or so collosi thought otherwise. :( 
 
I kept going and ran the remnant of my forces all over the map to make him earn his win. 

You only say GG when you are losing or after the loser has said GG.

I see were your coming from, that attitude of "might as well say it now cause imma woop your ass". In these situations I like to think that the accused meant well, keeps my faith in humanity going.

However, is winner gging better or worse than the loser not gging?
 
Also, for the mass Mutas try sticking just one or two Thors in your base. Splash + damage vs light lets them clean up mutas in just a couple salvos, and huge range lets them cover the entire mineral line without moving.
 
Edit: Never mind, you did Thors. Actaully, the anti-muta play looked pretty decent to me all around. One suggestion I'd make for backing up the Thors is adding in 3-4 uncloacked ghosts, so they can EMP/Snipe infestors and help deal some quick single-target damage to complement the Thors slower splash damage against the Mutas.

" All it would take after that is some zerglings to fuck me over if I Massed vikings. "

" @Cataphract1014 said:

" All it would take after that is some zerglings to fuck me over if I Massed vikings. "

You... you do know Vikings can land, right? And that they have a ground-to-ground attack once they do? "

A protoss player void ray rushed me and said "GG" when he entered  my base, declaring himself the winner.  But I held it off, and pulled out the sweet, sweet win.  The guy even said I was a lucky noob. What a scrub.  And I'm a zerg player so it was especially challenging to stop it.   
 
So yes, GGing to say that you win is BM. But saying it and then still losing means you are a terrible, terrible player.

" @Cataphract1014 said:

" All it would take after that is some zerglings to fuck me over if I Massed vikings. "

You... you do know Vikings can land, right? And that they have a ground-to-ground attack once they do? "

" @Fish_Face_McGee: Ah I need to put my glasses on! Either way, if that happens just use it as fuel to try harder and then give yourself the ultimate satisfaction of saying GG as you take the win from the guy. Turn that frown upside down! "

" Turrets make mutas useless.   Then just go do a few thor drops and wreck his economy.  And perhaps even Battlecruisers? I don't know how they fair vs. mutas. "

I had a Protoss player do the whole "I am going to put lots of photon cannons in your base"-thing. I responded by lifting all my buildings off and placed them elsewhere. Then he told me to give up, because I "had lost". That just made me even eager to beat his ass. A while (and another try of the aforementioned strategy) later he said something like "plz give up imma gonna get an achievemtz". I started builing marines, but suddenly he quit. In the statistics after the game, I realised he hadn't made a single unit, just pylons and cannons.

Ya, thats a sucking thing to do. BM indeed.

I always wait for the other guy to GG, but then he leaves so quickly that I can never actually say GG back :C