First of all, at the beginning of Half Life, you can see him ranting angrily at one of the Eli clones who was pushing the experiment to go ahead.Twilight_guy said:I think the G-man causes more evil then good. What with making sure black mesa blew up and manipulating Freeman to kill everyone and blow things up. He also acts far to much like a sinister monster. (Maybe he works for the devil?) Also, I doubt if the guys at Value would bring God into a video game.
Secondly, he doesn't manipulate Gordon at all in the first game. He, for whatever reasons, knows what will be happen next and watches Freemans action to see how he deals with these situations. Impressed, he "employs" Gordon as his or his organisations muscle to help spearhead the liberation of the human race.
The G-Man is someone who is realistic. He's looked at the overal goal and relises that sacrifices have to be made to benefit the greater good. It's not to do with religion but rather morality. As for who he is, it's impossible to say.