abbreviationfor the meme character Good Guy Greg, often used in the Reddit community to portray someone who does nice things but doesn't broadcast that fact for attention.
Dealer/GGG: That'll be 30 bucks.
Buyer: Hey man, do you have change for a second 20?
Dealer/GGG: Nah dude, just take it for 20.
Buyer: That guy's such a GGG!