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"