They LOOK extremely similiar looking, both of em have the same nose, green eyes, etc, maybe the G-Man is GOrdons's father or Gordon from the future, hence "G" for Gordon.
umm ok dude there are ,multiple threads on this tpic go find one of them and read it also check out the steam fourms for more topics related to this discussion.
17th June 2002
I think the name 'G-man' was originally coined because everyone thought he was working for the government. I was under the impression that 'G-man' was a term applied to any government agents. Now I think it stands for 'God-man', because of his ability to move through space and time without the aid of a donkey.
I didn't make it!
umm, actually the G-man was supposed to be Gordon Freeman....
But everyone though it was the guy in the blue suit so he ended up having the nick as the G-man..... they said that in a interview :D
I think both of them switched body, but mostly G-man, I think both G-man and Gordon are vorts and their goal is to save the race(Gordon has no idea though).