Couple things:
Ask yourself why it was important for you to go searching for this. Is it based on your own insecurity and you wanted to compare? Are you afraid she’s going to want this during your relationship?
She was 100% up front and honest with you. It’s not like you found this video by accident. She didn’t lie or deceive you or hide her sexual past from you.
All that being said, if it were me it would bother me as well and I understand what you’re feeling. Hearing about it is one thing, seeing it is a different animal entirely.
Definitely talk to her and explain how you are feeling. Apologize for the self sabotage and for fundamentally changing the relationship. I’m not saying end it, especially if you feel very strongly for each other. Maybe take some time for yourself to think about things. Will you be able to get over this and move forward? Do you think less of her and if so why?
Disclaimer: I’m a girl who has gangbang fantasies but that’s where it ends. I have absolutely no intentions of living that out. On that same token I don’t judge anyone who chooses to live out their own fantasies. Every single person has their own thing/kinks/personal wants. Good for her that she got to check this off her bucket list!