Could you edit the title to include a spoiler alert?
The G-Man definitely plays a pretty big role. After I beat the game, I started playing around with the no clipping cheat, and the gman is in several places of the game that you wouldnt see unless you had a sharp eye or were using the noclip cheat. Little clues, hinting that he is involved in a much bigger way that he at first appears. As for why hes helping you, it could be some sort of test... I dont know, but I suspect it will be revealed in due time