I'm upset that my gf got too drunk and woke up in a stranger's bed
What about the situation makes you feel hurt?
I don't like what she did, it makes me unhappy.
It is reasonable to be upset with your partner if she shows a lack of self-control and gets so drunk she sleeps in a stranger's bed. You have to decide if the relationship is worth repairing for you. You can break up with her over breaking your trust, or you can tell her she needs to start showing better judgement.
I know her well and we've been together for a long time. I think this is one time sexual encounter.
If you can forgive her, that is very mature. Just make sure you are truly ok with it.