Gabe will either come around or he won't. Right now he's pissed off and angry that you set a (VERY REASONABLE) boundary. He's having a tantrum.
Trust me, NO ONE knows where his head will be a week from now. Gabe is unsettled -- he's confused by OW pursuing him again (he gets a chance to be "right" with her and "win"), he's confused by running into a boundary with you, he's confused by his own feelings, including, no doubt, a whole helluva lot of self-hatred just at the moment because HE KNOWS D*MN WELL that interacting with OW is wrong, deceitful, and a bad way to treat both you and Marc. Make no mistake, despite his protestations, he knows full well he's being someone he doesn't like much.
In the meantime, take some space, respect yourself, treat yourself well. And, QUIT calling yourself stupid. Instead, recognize that you were very STRONG in choosing to open up emotionally while owning the risk. And, look how far you've come. Learning to do what you just did was essential for your future, no matter who it includes. Worst case, Gabe was a good subject for you to practice on, best case, there may yet be a surprise down the road.
You set your boundary. No need for drama over it. He can choose to respect it or not. While he's not, he doesn't get to be with you. Correct?
