I can relate exactly to how you feel--I was the "outcast" in my group because I was not into the same things most of the girls in my group; however, there was one girl in particular who would never really talk to me and when she did she always had something negative and rude to say. I generally tried to ignore her and just go in there and get my workouts done--I was there for gymnastics, not to socialize and be her friend. I also suggest "killing her with kindness" as MdGymMom01 said. Unfortunately some girls are that way, and hopefully she'll grow up. Something else that you can do is just try and respect her as a gymnast, while killing her with kindness, and show her how a real gymnast acts and respects everyone on their team--be the bigger person.
Most importantly do not give up and do not give in to her--you love gymnastics so go in there for gymnastics, not for making her happy. Good luck!