When someone is immune to using the letter "g" in their away messages, facebook comments, etc. (ie showerin off, eatin dinner, bikin), this exclamation is used between to outside parties to express mockery towards their dorkiness. There is a corresponding handshake, in which the outside parties slap their palms together, then the front of their hand, followed by a snap, at which time they exclaim, "G!"
howard: sorry i couldn't talk last night. i was talkin on the phone with stefi.
emily and simone: (handshake) G!