I think if he was open and honest about what he did with her that helps a lot more than if he's hiding it from you, but still... I would say a talk is in order, about how you feel and maybe trying some spur of the moment things with your boyfriend or see how he feels about that? Or maybe asking him to introduce you to Gina? I usually feel a lot better the more I hear about my boyfriend's friends or if I get to meet them.