Am I being paranoid and selfish if I don't want my gf to go to gatherings with the guy she cheated with from her past relationship
I don't think it's overly paranoid. Was she cheating on you or was she cheating on on someone else?
She was cheating on a guy from her past relationship.
Yeah, you should be pretty concerned if that's the person your girlfriend plans to hang out with. It sound's like a recipe for disaster and she will probably end up cheating on you with this guy. History is going to repeat itself.
I'm going to let it slide.
Well I think you're right to be concerned, but it's also good to show that you trust her. Just knowing that you trust her will make her feel comfortable with the relationship you have.