G-Man is a character in the half life series of video games but in super smash bro gmod he has a completely different personality.
In the Half-Life Series - Not much is known about him accept for the fact he guides Gordon Freeman. His intentions are currently unknown, but he is one of the only people Gordon can turn to in order to communicate.
In Supersmashbros Gmod's Films - G-Man is the mastermind of the first 3 movies, behind the defeat of many well-known heroes of the World of Trophies. His goal, like any other villain, is to take the world from the heroes and make it his own. He commands an army of well-known villains such as Bowser and Dr Eggman to achieve this goal. It is revealed in the end of the third movie that he was working as Giygas' right-hand man, and in return is greatly empowered with negative energy, trophifying any heroes that stepped up to him and Giygas. In the end he was ultimately defeated by Mario, who had defeated Giygas and weakened G-Man with his final smash, the Mario Finale. Once Giygas was gone, the only to breathing were Mario and G-Man. The two got into a fight of fists, the victor being Mario, who with one fatal punch trophified the enigmatic tyrant, ending his raid on the World of Trophies.
In the upcoming 4th (reboot) movie, it is revealed that he is not the only mastermind, but one of four; The others being Bowser, Eggman, and Neo Cortex. They are all commanding an army of many more villains and fiends than the last three movies to take the World of Trophies from its inhabitants.
In the Half-Life Series - Not much is known on what G-Man can do besides the fact that he can communicate with Gordon Freeman and appear in random situations.
In Supersmashbros Gmod's Films - G-Man is not shown to do much in the first 3 movies. He is a rather durable fist-fighter as shown in the 1st and 3rd films. Aside from that, when exposed to strong negative energy given off by Giygas, he becomes supercharged with many energized abilities, like shooting lasers and casting fire.
His abilities in the upcoming 4th (reboot) movie are yet to be known.