I play xbox with my girlfriends ex
Does your gf know that you are hanging with her ex?
No, I just want to ruin her day
It seems you are choosing to be a bad partner to your girlfriend. You should not betray her trust because this will lead to her resenting you and certainly cause the end of your relationship.
It's only a game, and I wanted to talk to someone.
Just tell your gf that you've been gaming with her ex. Set boundaries and never mention your current life with her ex. The less he knows, the better.