Like I said previously, probably in this thread, I'm going to disagree with you guys. Height matters. Gamora is going to be played by someone reasonably tall. Even if you don't want to accept it, it. It'll happen.
I'm going to stop touting Kunis but only because I think she is too busy. There is no reason why Gamora needs to be played by a tall actress, none. The difference between being 5'4" and 5'9" is really nothing that can't be overcome with the right boots and camera angles. That said, I do like the suggestion of Katrina Law, though I've never seen her act.