You sound like a very kind person who reaches out to others when there are successes in the gym, as well as when you see others hurting. I hope that you feel very proud of yourself for that, as I'm sure your parents are. I'm so sorry that this kindness isn't returned and think that the adults involved should be doing a much better job and have a zero tolerance policy for this outright meanness and bullying. I know that your gym is just a minute away and it's "good" in terms of gymnastics but it's not a positive environment for you or the other girls, for that matter. No one thrives emotionally in a place where that kind of lousy behavior is ignored.
I'm a counselor and have worked before in schools where this is happening. If there's not a willingness from the top down to confront this behavior, then it festers. If there's a small group of kids who go up through the grades (in a school) or the levels (in gymnastics) together, then it's hard to change kid's opinions or treatment of each other. You may be doing everything right and yet the others may not give you a chance unless the coaches and gym owner and parents make it very clear that this behavior won't be tolerated.
I'm concerned for you because it can be hard not to turn their ugly behavior into feeling bad about yourself, even though it has everything to do with their issues and not yours. Please make sure you keep reaching out, that if you feel lonely or bad, you reach out to friends and family and this forum so you don't feel so alone. No gym is "good" enough or convenient enough to disregard your emotional health. Think about seeing if you can -- behind the scenes -- join practice some other area gyms once or twice and see what you think. I think that could do several positive things for you: 1) show you what your possible alternatives are so they don't seem like scary or unknown options, 2) make you feel more powerful because you do have the power to change gyms, rather than feeling stuck with a lousy situation, 3) perhaps could put more pressure on your current gym to deal with the problem (you sound like a high-level gymnast and the gym should have a vested interest in making sure you are safe and being treated well so they don't lose you), and 4) if you don't like the other possible gyms, it will help you to focus your efforts on changing things in your current gym environment.
I know it feels like a huge risk to let your mom talk to their moms but I would let that happen. It could make things better but if it doesn't and gets worse, then you and your parents will have more clarity about switching gyms. You shouldn't have to shoulder all of this negativity by yourself.
Know that you have a whole community of people here rooting for you and supporting you. You deserve so much better.