Just horrible.You and your friends r**ed her.How do you wish to atone for this?By getting aroused?
OP,
Ignore #1. Drunk or not, she sounds like a willing participant. The big question is whether you're ever going to remind her of her drunken gangbang and try to parlay that into getting more sex from her. It sounds like she was a full service slut that night. Maybe she'd like a repeat with you using all of her holes again.
That was a different era ago. Different norms. Can't judge from today's lenses. When I was young my date came and grabbed me at a party and ushered me to a back room. There on the bed was her best friend. Her best friend was nude and passed out. My date was adamant that I screw her. I wasn't hard or in the mood for that kind of thing and let her know. When I asked why she claimed it was the Golden Rule. My date pleaded and then ended up being my fluffer. She swore she'd fulfil any fantasy of my choosing if I did this. I did what she asked but I must not have been enthusiastic enough as she never held up her end.