GTFO is an acronym which means "Get Yourself Begone", according to the NSFW episode Lulz & Order: Troll Court. It may also mean, "Get The Fudge Out", presumably if someone drops fudge on their clothing.
GTFO appears in many episodes of NSFW (usually via the soundboard as a robotic voice or a crowd yell), but was primarily featured during The GTFO Awards. From time to time, a clip of Penelope Brushwood shouting "G T F O!" from that episode is inserted into other episodes or the after show.