GP is a acronym for group poop or group pee. It is when a group of some kind dumps their load together. Most of the time during the group poop there are other activities the group participate in. For example talking, laughing, fighting, holding hands, kicking others while they are peeing in a urinal and making them pee on the wall etc.
Person 1: GP guys!!!
The boys: YESSSSS!!!
Person 1: I have a taser!
Person 2: Oh no.
Person 3: Iām not surprised but you have my interest.