Expression used when girls posting online try to gain more attention by stating the fact that they are girls even if it has no relevance to the matter at hand.
Tits or GTFO is used in a way to say; If you are going to use your gender to gain further attention on a board or comment section of a predominantly male user base, you are basically saying "I have tits, give me more attention". Therefore you might as well go all out and show your fucking tits or get the fuck out.
Girl: Hey guys, female poster here. How do you like this video/game/whatever?
Unfazed commenter: Tits or GTFO