Not this old cliche again *sighs*
Seriously, can't you consider a different theory. And his name isn't even gman, that's just his console script or whatever, and he was supposedly only given that name by the developers because he looks like a G-man.
Here is a plausible explanation I have developed that could explain G-man's nature.
Now, my theory, rather then having to do with some bs like "GMAN LOOK SIMILAR TO FREEMAN* and things like that, it rather has to do with the language he uses in his speeches to freeman.
Now what kind of talk...