You have 2 options

1. You leave.

2. You trust your GF.

From your post your GF hasn’t done anything to warrant any mistrust, in fact she seems to be doing everything perfectly. If she does do something to break the trust, then well option 1 is always there.

The only thing you really can do other than state again that you are uncomfortable with the situation, is to tell her that when he does something she doesn’t like, to not come to you to vent if she doesn’t want to hear ‘I told you so’ - yes this is petty, but you have told her, and while she has done everything right, and keeps telling him that she is in a relationship with you, he keeps trying. Sooner or later he is going to try something she doesn’t like.

Caveat to the ‘I told you so’ is if what he tries is assault, then you are in your GFs corner 100%. You do not say anything, do anything, or think anything that even hints that you are thinking about saying it.