Does anyone know how human beings became able to use portal technology? It is my own personal belief that the G-Man is actually the first contact with ETs, perhaps- an important member of a non-discussed super-advanced race brought down by the Combine, or Xen. He is known to have given the portal tech to black mesa in the beginning, and overseeing the project along with at least visiting and
speaking to all of the most important people in the project (the scientists and the people responsible for cleaning up the mess if it gets out of hand.) When it comes to the main character's survival, it seems to me that he intentionally helps the player out in every instance that he can, either by watching and making sure at certain intervals that you are okay, and by helping you out if not (I think he just SEEMS creepy because he isn't human) . However, his own survival is paramount to him, so he works behind the scenes, staying out of harm's way. Once black mesa was botched (probably because of inconsistencies in the prototype tech made by the humans who wanted to use the crystal that the gman supplies you), he tries to rearm the bomb that will destroy black mesa, thereby reducing the number of alien portals that will open up, and attempt to save the primitive human race from war with an entire multiverse full of hostile aliens. I'm thinking that XEN was the opposing race of the Gmen, or at least his employers hence the "Xen is now under our control" comment.
In short, Gman is first contact one, an ambassador from a super-advanced alien race, probably from another dimension, whom deemed human beings worthy to wield more advanced technology because they are significantly less barbaric or cruel than many other races in the multiverse. Gman was sent as an overseer to the inaugural event, to make sure everything went according to plan. when black mesa failed, and the xen invasion begun, he had to work as swiftly as possible to ensure that this event did not destroy the human race which his employers, the watchers, sensed great potential in. Because of the portal storm, the combine took a foothold on the planet, then sent through a fleet, and because of their superior advanced technology, took over the planet quite quickly. The gman, sensing great potential in gordon, wished to make him a valuable knight on the chessboard for defeating perhaps an enemy of the gman's race (which i will henceforth refer to as "the watchers"), the combine. Ugh i would type more but you get the general idea. Im so sleepy!
He is obviously either alien or some kind of advanced android due to being unable to speak properly. I also do not believe that he is sane, or at least cannot think in the same way that humans do.