GUPO: This is code for straight girls to describe that super hot guy at the pub who has clean fingernails, is wearing a pink floral shirt BUT is drinking beer from a can. 'Do you think he'll ask me out?' 'Be careful, he's definitely GUPO'.
Gay Until Proven Otherwhise. Someone who hasn't been seen dating anyone of the opposite sex for quite some time/ever. Friends or colleagues will assume that such a person is gay until they have seen "proof" to the contrary. Mostly used about women.
Derived from the thumb rule of justice: "Guilty until proven otherwhise."
Oh, you know, nothing much - so the usual. She's a GUPO if I've ever seen one.