I want to tell my girlfriend to stop telling me when she talks to others about me
Why don't you want to know when she talks about you?
It makes me uncomfortable to know she's talking about me.
You should listen to your girlfriend when she talks to you about you. She may like talking to you about it and seeing your reaction. This is a normal part of a relationship.
I think I'll handle it in a mature way by being mean to her. Maybe she'll stop talking about me to her friends if I act like a kid.
Acting like a kid is not a mature way to handle this. You should become more comfortable with her talking about you to others. You can't control what she talks about.