It was made popular by the cast of Jersey Shore on MTV.
Use: This can be used when leaving a conversation, Saying you have to run errands, or things to do.
Person 1: "Jersey Shore is Pathetic, why do people watch this show."
Person 2: "i know right. Its so stupid, but it makes me want to fist pump."
Person 1: "haha agreed. well i gotta GTL."
Person 2: "later"