whats up with G-man
Suicid3: "I honestly don't even remember seeing the G-man in Half-Life 2 lol Except for in the cinematic of course."
There are eight different places to find the G-Man in HL2.
As to who he is, he's an intergalactic bureaucrat. He was the one who delivered the Xenian crystal sample that caused the resonance cascade in HL1, engineering the events that triggered the Xenian occupation. HL1 ends with the G-Man stating that his mysterious "employers" have taken control of the border world, which is the hub for all intergalactic travel between universes. It's also the only way to use local teleportation, which is why the Combine want it so badly.
At the end of Half-Life, you're recruited by the G-Man.
In HL2, it's hinted at that the resistance bartered something for your "awakening;" the G-Man was selling your services to the highest bidder. Of course, he also states that there were certain offers he never considered taking until AFTER the events at the citadel, so whether something was actually offered in trade, or whether he loaned you out to defeat the Combine for his own ends has yet to be seen. This makes him something like your manager, and you his (unwitting) agent for hire.
As of Episode 2, it seems like he wants you to go to the Borealis and take the portal technology developed by Aperture Science, which is why Eli Vance was so adamant about you destroying it. This will be the plot of Episode 3.
So we know from interactions that the G-Man is not a figment of Gordon's imagination. After that everybody's got their own theories. Me personally, I believe that the "employers" he's mentioned on several occasions, the ones who impose restrictions on what he can and cannot do, and those he even had to get clearance from to hire Freeman in the first place, they're likely a competing faction with the Combine, G-Man possibly being Earth's representative...in the G-Man's .cpp file there's a comment line that reads, "//Misunderstood servant of the people." I don't buy into that "He's an alien in a host body" shit. It's too cliche. But who knows?