From everything you have shared, it really sounds like your ODD loves gymnastics, is a very hard worker, and you have moved mountains to keep her doing what she loves and give her opportunities that are (sadly) increasingly fleeting due to her not being super young. There will always be mean girls/people, and there will always be messages about our body size/weight from a million different sources and angles. It would be a shame to retreat from her one opportunity because of the shortcomings of others (i.e., some meanness/insensitivity of 1-2 girls, a somewhat apathetic coach).
No easy task, but I suppose I'm more of the mind to try to thicken the skin over time - to insert oneself into environments that may repel you at first, rather than retreat away from potential clashing and sacrifice your own wants and needs.
I have a child who has started to be teased for his size, and will likely be more perpetually teased as he reaches middle and high school. I do the best I can to teach him to respond to negative comments with calm, inquisitive, matter-of-fact-style conversation - to face it head on, not by retreating. It might not always work, but I try to teach him to "hold open the doors" with other people, even when they are mean or insensitive, and perhaps some will come in eventually. And some are just mean and won't ever come in, but the idea is to set him up as the person in power - the one actively holding the door for others - to change the mindset from victim to owner in charge of his own "house" and who may come in. He also has the power to decide to close the door to anyone who has crossed the line too many times. I could go on, but you get the idea. Basically I can't protect him from all situations where he will be bullied, but I can give him tools, and try to put him in the proper mindset, so that the bullying doesn't take hold and chase him away from places and things he enjoys. At least that is the hope
If my daughter were getting these comments, I might teach her to respond with things like...
Girl: Ugh, you're so big and heavy! Why do you have to be my partner?
DD: Spotting me probably is challenging. I'm like a giant, right?
DD: Yep, all that muscle is super heavy. If it's really too hard for you, do you need to ask the coach to switch?
DD: I know! You're like a tiny butterfly compared to me.
I need a bigger partner so I can actually build some muscle spotting you.
DD: Ok, Susie, I've heard you say that a few times now. I'm obviously taller and therefore heavier. I know that's hard for you. If you really don't want to work together, lets talk to the coach about it, because I want to train and I need a supportive partner.
Maybe she already responds in a supportive way, and just gets eye rolls or more hostility? But if she isn't responding at all and just letting the comments get under her skin, maybe it's time to amp up the skin-toughening?
I am so sorry she's running into this in her gym. I hope it is just 1-2 girls, and they can come around or she can learn to tolerate their insensitivities. I would hate to see her chased away.