Here are some theories that I’ve heard:
First off, there’s the theory that Gman is Gordon Freeman :freeman:. The evidence: Gman kind of looks like Gordon, and their names are similar: G(ordon Free)man. Evidence against: Gman is a fan name, not made by Valve. The name Gman came about when someone said he was a Government Man, and the name stuck. Secondly, The pictures showing how Gman is similar to Gordon are all faked. Plus, Gman has the distinctive pointed hairline, which is a recessive gene, and Gordon has a straight hairline, which is dominant. No matter how old you get, your genes won’t change. You might say: "well gordons bin around radiation n tht mite muteate his genes so. tht also explainz th superpowerz. 
" Well here’s news for you: you don’t get superpowers from radiation, and the odds of any mutation that leads to something other than cancer are very small, the odds that the mutation is beneficial are even smaller, and the odds that the mutation gives you anything resembling superpowers are UTTERLY ASTRONOMICALLY IMPROBABLE.
Another Gman theory is that Gman is the leader of the Combine. The reasons: He doesn’t seem fazed by them, and he often appears in Combine territory. He destroys Black Mesa as seen in OF, and he seems to be recruiting people with extraordinary survival abilities (as if for an army) such as Freeman and Shephard. I don’t have much evidence against this one, but I just don’t have any idea why he would unleash Gordon on his own soldiers and such, blowing up the Citadel and closing the Combine Portal, other than “training.” I just don’t see why this would be true, except for people thinking: “GMAN IS CREEPY SO HE’S BAAAAD!!!11!!111one111!!”
One of the lesser known theories is that Gman is GLaDOS “Because they both guide you through the game.” Evidence against: This is stupid. I’m not even going to start listing reasons why this won’t work.
My theory (I’m not saying I’m sticking with this one, though. This is just one of the ideas I have): Gman is the Nihilanth, or one of the Nihilanth’s lost race, that went into a host body once it died. Reasons: The Vortigaunts seem to hate him, as seen in the opening cutscene of Episode 1. He teleports all over the place, as if he were using those green portals. He can control and manipulate people, such as Gordon and Alyx, like mind control. He talks like the Nihilanth, smoothly and slowly, and talks in the same tone of voice. If there’s any evidence against this, please tell me.
