This made me think a bit. First I thought "gg" is the internet equivalent of a cashier saying "Have a great day!" They may not mean it but they say it to be nice, and you can't really fault them for that. Then I realized it's the internet so 90% of the time the fuckers are doing it to be assholes and they wouldn't dare say that shit if they were in ass-kicking range.
But I don't really say anything in games unless someone mentions me, if it's positive I'll thank them if it's negative I'll start a blood feud.