There is a girl which has shown IOI's to me and I like her too. I'm confused now, because I can't treat her like a gf, but I can't act indifferent and disinterested.
If there is a situation where I have two choices - do something really nice for her, or treat her like everybody else (we haven't had sex yet). Which option should I pick? (Let's say there isn't a middle ground)
If I do first, I can't figure out if I would be a gentleman or just an AFC nice guy. However if I do second she might think I'm not interested.

This is just wrong, dude... And you're being really ambiguous with the info you're providing...

Are you dating her or not?

Have you been out with her or not?

Is there any reason at all to even assume she's interested in sex, beyond a couple IOIs you think you got, or are you simply admiring this girl from afar?

Is there any particular reason why you either HAVE to do something nice for her, or you HAVE to treat her like everyone else? That's just fucking retarded.

There are WAY more than 2 choices... and if there isn't, neither of your choices are good.

You treat her like the girl you're interested in... and you play the game.