2 theories
1: it stands for Gaylord like in Meet the Parents and he prefers 'G' for some reason.
2: it stands for Goldman but someone else has that name and people kept getting confused:
'We have to go defeat Goldman!'
'Which Goldman?'
'The evil one with the zombies, obviously!'
'Oh. That's confusing.'
'Well, not really...'
'Oh, I'M sorry if I don't possess the crazy psychic powers necessary to tell which Goldman you mean! Jeez! Maybe we should just call the non evil one G.'
'...fine whatever.'
Sorry I got a bit carried away.