Technically you're not lying if the game was good or not, it simply does not matter, to me at least. I believe "gg" is acknowledgement of your opponent's skill or whatever and you were privileged to play with them. Regardless of your victory or loss, it's common courtesy. Like everyone has pointed out, it's your choice whether you wish to say it. I almost never say it unless it really was a good game or if the opponent says it. Know this, if this wasn't kept a tradition amongst gamers, there would be nothing to string us together as a whole, no?