This is an abbreviation for "
grandpa cock connoisseur". This refers to someone (usually female) who prefers the penis of older men, potentially elderly and senile. This girl is in her early twenties and has had a series of daddy
issues she is trying to solve by sleeping with men her dads
age.
Dude 1: "dude what's with
maggie?"
Dude 2: "IDK man, she's a major G.C.C."
Dude 1: "a what?"
Dude 2: "a Grandpa Cock Connoisseur"
Dude 1: "oh yeah, she totally likes
old wiener"