A guy who is obviously and undeniably gay.
Usually used to alert another dude who's blind to the clues and doesn't realize is being picked up by him.
John: Hey Dan, have you met Tim? He's such a cool guy.
Dan: Is that the guy in your class?
John: Yeah! He and his roommates are throwing a party at his place and he said we're invited.
Dan: Dude, this guy is gay gay as a fish. If I were you, I'd watch my back.