While I agree about the prisoners, to be far, Quill wasn't just an ordinary human at the time, so it makes sense that he'd be able to contend with her. The prisoners, on the other hand, should've all been slaughtered by her with relative ease. But I guess they needed to downplay her ferocity so that she and Drax could have their little moment, which is lame, of course.
As for the Nebula ordeal in Vol. 2, you do have to remember that by that point Gamora was extremely tired as they had already been battling for a few minutes by then.
You are right though, Gamora's level of power has been fairly inconsistent, I'd like to think this will be rectified going forward but we shall see.