I'm not going to go to my friends get together.
Why don't you want to go to your friends get together?
I have to go out with my gf, don't want to go actually.
It is fine to go hang out with your girlfriend. If you don't want to go, then don't go.
I will tell my friends right now.
It is good to tell your friends up front if you are not going to something. Your friends will understand if you tell them why you are not going.