Title says it all. There theories I'll explain have been carefully thought.
'''First theory alien'''
The G-Man is probably an extraterrestrial being whose employers and goals remain unclear. He's shown interest in Gordon Freeman since before the Resonance Cascade (he can be seen twice before it happens) and has helped him ocasionally. In exchange, Gordon does some jobs for him (killing the Nihilant and defeating the Combine).
This is the theory I'm more inclined to believe as it makes sense with Half-Life 1 and 2 and I haven't found evidence against it.
'''Second theory hallucination'''
Gordon Freeman imagined everything that happened after the experiment. He received a radiation discharge which put him in a catatonic state (or something like that...) and made him imagine the Resonance Cascade and the alien invasion.
It would make sense that players didn't think of this so it was retconned in Half-Life 2, which hints nothing that supports this theory.
The sources for this theory would be
A)The scene before entering the Nihilanth's chamber
"It is interesting to note that at this portal to the game's last battle, the voices of scientists that Gordon has previously encountered throughout the game, including some not even heard at that time, such as the voice of a scientist "Get him out of there! Shut down the equipment and someone get him out!", can be faintly heard, although hard to hear in-game through the alien sound effect around it, with no proper explanation as to why. Note that the altar looks like a warped version of the Anti-Mass Spectrometer chamber." http//img196.imageshack.us/img196/6486/38578537.jpg
B)The video "http//www.youtube.com/watch?v=0ZBwwo5olk0 Psyche"
The only image that appears only once says http//www.halflife2.net/forums/archive/index.php/t-20196.html some interesting things and the video finishes with the interesting quote "Well, well. Isn't this just like old times?", a possible reference to the possible retcon.
Many theories are that The G-Man is Gordon's father or Gordon from the future. This is mostly based on people believing that "gman" stands for "'''G'''ordonfree'''MAN'''" which is incorrect since it's actually the slang term for "goverment man". Also, any resemblance between Gordon Freeman and The G-Man are most likely as a resulted of in-game character-model similarities rather than any implied family bond.
So, what do you think?--UserUltraViolento|UltraViolento 1730, July 11, 2011 (UTC)
If the second theory is true fuck you, Valve! Worst. Plot twist. Ever. UserBramblepath|Bramblepath 1944, July 11, 2011 (UTC)
Personally, I am pretty certain that the first option is the correct one, seeing that nothing is against it. I personally believe that G-man is a supernatural alien being, that are just capable of taking the shape as a human, probably to familiarize himself with Freeman and Adrian Shephard, making him more comfortable to them. I personally believe, that due to him mentioning that his employers is in control of Xen after Freeman defeats the Nihilanth, and based on the evidence that he delivered the crystal triggering the Resonance Cascade, as well as that he appearently warned the HECU about their upcoming Black Mesa mission before the incident even happened, as evidence that he was planning to cause an invasion of Earth, and then use Gordon Freeman to defeat the Nihilanth, allowing his employers to take control of Xen, before hiring his expertise. Shephard was just a result of G-man predicting and admiring his capabilities, and then detaining him contrary his employers wishes. The reason for rescuing Alyx was probably because the G-man was capable of foreseeing her importance in defeating the Combine two decades later. He then kept Freeman in stasis until he could use him, and although his interest in defeating the Combine is unknown, it is obvious that he wants to use Gordon to defeat them, at least on the Earth, until the Vortigaunts take control away from him. I think the G-man = Gordon freeman thing is ridiculous and baseless, and as Bramblepath said, it would suck that bad that the plot of Half-Life would get on the same intellectual level as that of tetris. Fortunately, this is highly unlikely.--UserFijure|Fijure 1952, July 11, 2011 (UTC)
I also prefer the first theory and I agree with everything you two said except that I prefer to think about Gearbox expansions as non-canon mods except for Black Mesa's destruction and some other details. I also think that the second theory being retconed is a posibility as the 2 things I pointed out can't be ignored.--UserUltraViolento|UltraViolento 2021, July 11, 2011 (UTC)
The Gearbox games are officially canon, so we have to take them into account. But personally I don't think we'll ever get an answer to this question. In fact, I hope we don't! UserBramblepath|Bramblepath 2029, July 11, 2011 (UTC)
I would also be fine off without a final decisive answer, but I think the game is most likely pointing towards the top reason, for the reasons mentioned above. I am considering the Gearbox games canon unless they directly contradict based facts, which I don't see they do, and those games only support hte first theory, seeing his involvement in Opposing Force. But as Bramblepath mentioned What amkes the G-Man such an interesting character, is that he is that unknown an ambiguous as he is. If everything about him added up, I don't think he would be that popular. Now, I'm the type that likes if everything adds up in a piece of fiction, so I'm not particularly fond of the G-Man character, he seems to much like a God to me, but he would be even worse without being mysterious.--UserFijure|Fijure 2119, July 11, 2011 (UTC)
Gearbox expansions are official, but Valve had almost no involvement in the development the games (Gearbox Software of the Half-Life expansions) and I'm fairly sure that when Valve developed Half-Life they weren't counting on an expansion, which could support the second theory a bit more, even if I prefer the first one.--UserUltraViolento|UltraViolento 0157, July 12, 2011 (UTC)
I just played the portal to the Nihilanth part again. If you save the game it appears as "XEN" on the saved games list even though you are supposed to be on the "Interloper" chapter. The voices that are heard are random scientist voices from throughout the game and the unused sentence is probably just a coincidence and has no further meaning. Maybe the meaning of that part is that the Nihilanth is showing you its telepathy. If we asume that the resemblance beetween the portal and the Anti-Mass Spectrometer is just a coincidence, we could almost rule out the second theory except for the video. I still believe that both theories are true, the second one being retconned as I explained.--UserUltraViolento|UltraViolento 0133, July 15, 2011 (UTC)
My beleif is that originally the hallucination therory was the original idea behind it, however due to its popularity and gearboxes request to make expasnsions , the sequels, and the confirmation of the destruction of black mesa likely cancels that theory out, there is little evidence that the G-man knew of Freeman before his employers so it is unlikely that it is Freeman from the future or his father, therefore the beleif that he is an interdimentional being makes the most sense, it is likey that Valve will never clarify this to further add to the mystery.
The second theory is right, Gordon is a hippie smoking weed along with Barney, Adrian, Eli and other scientists (no, I didn't smoke weed to write this comment, I don't smoke at all). About Psyche theory I fixed the Psyche map for Source Engine 2007, and G-Man appears to be either afroamerican or man in black, http//steamcommunity.com/id/SiPlus/screenshot/541777522589447560/ proof. UserSiPlus|SiPlus 1726, September 5, 2011 (UTC)
Well, you must have done something wrong. But still, that explains why The G-Man isn't black he would be invisible on many parts. And I've never seen anyone black with green eyes...--UserUltraViolento|UltraViolento 1734, September 5, 2011 (UTC)
This is shader problem in vmt file. 11 colons... I hate stairs. UserSiPlus|SiPlus 0428, September 6, 2011 (UTC)
He can't be a hallucination. For one thing other people (Shepard, Eli, and the scientist talking to him in Anomolous Materials) see him how could other people see his hallucination? and also how could a hallucination stop time or teleport Freeman? SpecialContributions/220.127.116.11|18.104.22.168 2019, October 2, 2011 (UTC)