the phrase "G" derives from gamerscore addicts. Achievement addicts or "achievement jockey's" use the term G as a short name for gamerscore. These people often shout G in an aggresive manner when unlocking achievements, some have even been known to scream the phrase in normal life. these bursts of G are often followed by a mounting by the person, this often leaves the vicitm parylised for days.
Jack: "look at that, you got 30 gamerscore for that achievment, fuck yeah!"
Matt: "G G G, i need to mount something like a fucking whore" 'the jockey then begins mounting the nearest person or object'