I'm going to hook up with someone I didn't know being in a relationship and I won't say anything to the gf
I'm taken aback. Why would you still pursue someone in a relationship?
I don't want to tell her as I am interested with the person that is in a relationship.
Cheating is wrong and I feel like you will be an instrument of that if you continue to have this relationship. I suggest that you do not pursue this person and continue finding people that are single.
But we will just do it from time to time.
It doesn't matter if it is just an occasional hook up, it's still cheating. You can hook up with people that are single, and just not do it with people that are in a relationship as if they can do it with someone else, then they can also hurt you in the future.