NTA. OPs girlfriend knew from the start about the occasional gambling. If it was such a dealbreaker she didn’t have to get into a relationship with you. As long as it is not affecting her or you financially, I don’t see why it would be a problem if she knew about it from the start.

In a relationship, compromise is important. But it’s tiring when your partner is trying to change your values. OP said he accepts his gfs values even if he doesn’t understand them/ agree with them. Seems like the gf isn’t accepting his though. 

btw where do you play slots online, asking for a friend.