GGG stands for Good, Giving, and Game. More specifically: good in bed, giving equal time and equal pleasure, and game for anything—within reason. The term was coined by Dan Savage, gay author and sex advice columnist for the Stranger.
The opposite would be someone who is shitty, selfish, and totally square in the sack.
I thought my partner would be reserved about my kinks, but he turned out to be GGG.