Old Game, it is literally the acronym from a game called "Galactic Conquest" I played back in middle school (6th or 7th grade). There was a chat box that also mixed with its far more popular sister game "Unification Wars" (It spammed nearly every site with ads for it back then). When I registered for Unification Wars "Underlord" was already taken so I added the GC so people would know I'm the Underlord from Galactic Conquest and that was that. Although it would be another year or so before I stopped using underlord completely and solely started using Underlordgc.
EDIT: Holy shit I even remembered my old password and reactivated my account >.>