It sounds like the girl acted like a jerk, but I have to jump in here and say it is extremely annoying when they make you gate check a bag that you know will fit without any struggle whatsoever. Especially when they're making you gate check a very small, thin suitcase simply because it has wheels but they're letting people through with massive backpacks and duffle bags that have no hope of fitting. I admit that I have been guilty of taking the pink tag without argument, slipping it into my pocket, and bringing the bag on anyway. I have also been guilty of assuring the gate agent that it fits, which is effective when I name the type of aircraft we're flying on, like, "Oh, I bring this on the CRJ-900 every week. I promise it fits." I don't know what kind of bag she had, but if she really did know it would fit and had taken in on that aircraft a ton of times before, I understand her annoyance. That doesn't excuse causing a big scene at the gate and on the plane, though.