I want to play my ukulele against my gfs wishes
Why wouldn't your girlfriend want you to play your Ukulele? What would happen if you did?
I want to be annoying, she treats me quite poorly sometimes. Otherwise I have no reason.
I'm sorry that your girlfriend isn't the best sometimes, but you still need to respect her instead of retaliating. Forgiveness is better than revenge, and you should treat her kindly and learn to work out your differences.
I'd rather have revenge. It gives me a sick pleasure.
Retaliating will only make her upset and she may feel the need to do the same. Instead why not just find a place away from her you can enjoy your music? Maybe even soundproof a room so you can practice?