What is my gf cheated on me with another girl?

I'm sorry to hear that your girlfriend cheated on you with another girl. This situation can be emotionally challenging to deal with, and it's important to take care of yourself during this time. It's also important to remember that cheating is a breach of trust in any relationship, regardless of gender. You may want to consider having an honest conversation with your girlfriend about what happened and how you feel. Ultimately, the decision of whether or not to continue the relationship is up to you, and it's important to think about what is best for your own emotional well-being.