I'm going to hide my friendship from my gf
I can't see that ending well. Why would you hide your friendship from your girlfriend ?
She doesn't like this person I hang out with.
You shouldn't hide things from your partner, but she also shouldn't control who you are friends with. Talk to her and tell her the truth.
I'm going to do that. Haha! Gotta admit, you are supporting what I believe in.
I think you're making the right choice here. Being honest and open with her should make your relationship stronger.