I have found GG to be completely ineffectual coming from other people. It is just letters to me now. I remember it used to be a bit more meaningful when we first started seeing it 9 or 10 years back. Now it is just nothing. A reaction people have to a game ending. If a game is truly good, people usually type something more about it. i.e., "Wow... you guys totally kicked our arses." or "Holy crap, good job crew. Don't know what I would have done without you," etc.
However, when I say it, it is usually a way of coping with anger at my own crew or my own failure to play right. Kind of like sucking it up so I don't get upset at a video game. i.e., Got no kills. Died 4 times. Tried to fly a Galleon., "GG."