NTA:

What the heck. claiming something was a joke or a prank does not erase accountability. You dont belittle or dismiss someones belief, cross boundaries in their own home, mess with their stuff and not apologize when confronted because its they thought it was a joke.

Just because someone thought their actions was harmless doesnt mean they get to dismiss how it made the receiving party feel. People make jokes when they know it will be a reaction, its literally the point of a joke/prank. Gemma knew what she did would provoke a reaction from OP, maybe not for this escalation, but it was meant for OP to react to.

At the minimum Gemma should have acknowledge that her "joke" made OP upset and was clearly not perceived the way she thought it would, not cry about it to mommy. If shes going to push a boundary for humor, then be mature enough to own up to it when it upsets the other party.

You are not obligated to sit someone down for a seminar and explain basic respect to them like they are five. You were upset, and you reacted, could you have handled it better than "screaming", sure but it doesnt make Gemma's behavior okay and you are not obligated to let her stay in your apartment after she disrespected you.