I was playing through the Orange Box today, specifically Half-Life 2 and it made me wonder... 
 
Just who exactly is the G-Man and what is his purpose in Half-Life? I know he is the creation of many funny and stupid poses in Garrys Mod.  But does he have a purpose?
 
I have not yet played through Episode 2 fully, so i was wondering if they explain anything at all about him throughout the series. Or do you think he will be explained more in Episode 3?
 
What do you think is the story behind him?

i think he is fecking creepy

He's Gordon's employer.

G-Man is the man! That's all you need to know actually.

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That is a age old question people have been asking for a long time.

If Episode III is ever released, you might find out. Probably not though.
 
In conclusion: We aren't supposed to know so you aren't going to get any valid answers. Just take the "wait and see" approach.

I've heard speculation that he is Gordon from the future

I've heard speculation from tonnes of people about tonnes of different, probably wrong guesses.

There has been tons and tons and tons of speculation on the g-man.
 
As far as I know, this is the most complete and accurate explanation of what we know.

At this point his character is so hyped up that the reveal of who he really is will probably be lame.

There is never going to be a g-man "reveal". Valve hasn't built up his character like some sort of Andrew Ryan, it's not like there's suddenly gonna be something like OH MY GOD THE GMAN IS YOUR FATHER. Or... YOU ARE THE GMAN.
The gman is a plot device, more than anything. A very effective, insidious one. His origins and purpose will only be alluded to, but full disclosure will never happen.

HE IS THE GORDON-MAN.  
OH GOD

We will never know..

Why bump this with such a useless post.
 
Anyway, I don't think Valve plan to tell the whole story about him even in HL3. Any explanation would be really contrived and make no sense at all anyway.

He is God damn it

G-man is in many ways the only thing keeping the Half-Life saga alive, as he can control time and space and force G. Freeman to act accordingly. 

I don't know but I really like the theory that he is Gordon from the future. How crazy would that be to find out the guy that has been pissing you off for so long turns out to be you from the future. Then you realize you have to grow up and be this d*** in order to save the world.

no one knows who he is and i doubt anyone ever will. Even if more half-life games are released i dont think theyll ever reveal who he is or what he does. 

Gordon Freeman  = gordon freeman 
 

I think the simillarity between Gordon and the Gman are indeed intentional. But maybe valve are not planning on pulling a regular-ass plot twist like everyone is thinking. Maybe Gman assumed the form of Gordon´s father, so that he can control him better/ seem less frightening. Of course he´s still hella frightening with the way he talks, but Gordon does not seem to be afraid of him.
 
I imagine the big reveal to be something like the end of the movie "Contact", where the alien says his real form would be too much to comprehend for a regular human. Another theory suggests that he´s not actually a real person but much rather a product of Gordon´s imagination. Now, Gordon doesn´t really seem like a sane person to me (he NEVER TALKS), but in HL1 you can see the Gman talking to other people around Black Mesa. 
 
Above all, the Gman is a very clever plot device and his speech at the beginning of HL2 is the perfect way to introduce you to the fucked up place earth has become.

He's Gordon Freeman's brother, Gordon Greenman, they call him G-Man for short

Gordon's Zach?

i like to think that freeman is just messed up in the head. Freeman iis schizo, G man is a part of imagination like an inner 3rd person  monologue. Freeman = G-Man. Can anyone else see that ?