There's no need to have an understanding that one couple who practice poly will tell each other EVERYTHING. Good communication is important. Honesty and transparency about one's own feelings and decisions that affect your partner(s) is important.
But a hinge should be tactful. And there are things one partner says or does that a hinge shouldn't share with her other partner(s). Hell, even mono people won't tell their mom everything their lover says or does. They might tell their bff something intimate about their lover or spouse, but that can get sticky. I know I've felt uncomfortable around friends' husbands because my friend told me intimate or unpleasant things about him.
What your gf's OSO says or does should be between them. Telling each other EVERYTHING only concerns what you two personally think or feel.
Your gf should be asking her OSO for consent about what is OK, or not, to share with you, just as she should be asking you what is OK for her to share with him. Maybe he wouldn't find you gross if she wasn't oversharing about you with him.
Telling you that he finds the idea of her fucking you nauseating did no one any good. It wasn't helpful. If anything, it should make her rethink how much to tell you about her other relationships. You didn't need to hear it. If anything, she could have said something more general, like, "My OSO struggles with the idea that I am poly (and so maybe he's not right for me)."