There are many theories, for example, he is future gordon who came back to help himself (why? dunno :S) who is he?
I think the most compelling evidence from discussions I see lead me to believe he's future Gordon. That, or he's some weird... alien... guy... thing.
The vortigons see him. Eli knows about him. The first or second time you ever meet him (in Mesa) he's in a room, talking to a guy. He supplied Black Mesa with the crystal thing that Gordon started the whole resonance cascade with. How could G-Man possibly only be part of his consciousness? More like an incarnation of it And a future version of himself? Tell me some of the arguments Rojo, cause that just seems silly.
Think Fight Club. Fenix's idea is actually feasible. I completed HL1, Opposing Force, and Blue Shift, but I really should play them again and then HL2 because the story is good and I like the mystery around the plot and this gman. But time... ugh.
Here is my theory about G-man, simply put he is a "representative" for another alien species out there that is of yet unknown. I believe he is using Gordon Freeman and his other influences on earth such as the government and black mesa to further this races agenda. Firstly he was the main reason behind the black mesa incident by crashing the Black Mesa servers and introducing an unknown material to the experiment that caused the rift. This implies that he has knowledge and objectives beyond that of humans. Then there is just his personality, his speech, how he just happens to be at the right place at the right time and access to technology he has. I mean he is able to suspend Freeman until he is needed and teleport him where he wants him for the most part. I highly doubt he is human, I think he just somehow takes a human form to blend in more with humanity etc and not reveal what he is really is.
I think it would be a nice plot twist if G-man was Gordon's father. But I think he's not related to him in such way and is just a bureaucrat of some galactic organization working against the Combine. I bet he's not even human but simply taking that form to appear more familiar. So he uses poor Gordon as his tool and dispatches him in crucial times to fight combine, and when he's done packs the guy up in stasis field again :laugh: LOL Mong0! "I am your father" line would be epic