I've been going through the game fairly smoothly with my current team, but as I approach the 8th gym, I'm realizing that I've got some potentially serious gaps. Does anyone have any recommendations for a 6th Pokemon, or suggestions to replace my 4th or 5th?

My current team:

1. Greninja: Water/Dark, uses water, psychic, rock, and normal moves
2. Charizard: Fire/Flying, uses normal, fire, and flying moves
3. Gogoat: Grass/(maybe ground?), uses normal, grass, and ground moves
4. Bibarel: Normal, because he can use dig, cut, rock smash, and surf
5. Sigilyph: Psychic/Flying, uses psychic and flying moves (but he's mostly in my team just so I can use Fly)
6. I swap out the 6th if I need something specific, but usually I have Pangoro: Fighting/Dark, who uses normal and fighting (but no dark) moves. He's currently the one I have that can use Strength.

So I think my biggest weaknesses are against fighting, dark, and maybe psychic(?) but I'm pretty sure a moderately strong flying/electric would also tear my entire team apart. Anyone have any suggestions for something that could cover my current weaknesses? I'm playing X, by the way.

Your biggest vulnerability is definitely Electric, followed by smaller vulnerabilities to Ice, Rock, Bug, Fighting and Grass.

The 8th Gym is Ice, so you could base your 6th on that, but honestly nothing in the main game should give you much trouble thanks to the Exp Share. Unless you are severely under leveled, my advice is always to just pick something you like. The E4 doesn't have any of your weaknesses as their main types.

That said, I'd go with a Ground type for #6 I guess?

I would get rid of Bibarrel and or Sigillyph. There are only so many places you need surf and fly is totally useless in the field. The best HM slave in the game is Lampras, it can literally learn all of them except Cut and Fly. You just give them all to Lampras and keep 1 flying type in your box with Fly and teach Cut to one you use. You take the Fly pokemon out when you need to travel (it is only from pokecenter to pokecenter anyway) and otherwise sits in the box. You need HM's other than Cut you put Lampras in your team until you are done. Remember, when you move a stone with strength it's over, you moved it, it will stay moved the entire game you never need to do it again.

Go catch yourself a gible and get to leveling it. Once it turns into Garchomp it is just a brutal pokemon and with the Earthquake TM it will dominate a ton of fights. Then get yourself a Fairy pokemon, with your team make up preferably a Gardevoir. This covers the psychic and fairy movesets. Pangoro is fine, you just need to get him some dark moves, check your TM's there are decent moves there he can probably learn.

Make any other changes as needed based on your legendary. Xerneas is obviously superior to Gardevoir for example or Yveltal for Pangoro.

6 pidgeys. GG