Very well said. That is where I am at aswell. I dont actually worry about her falling behind I worry about the message it is sending.
I think the message depends on how this situation is presented to your child. "Your practice schedule changed to during school one day a week, I tried to get them to reconsider, but they won't. This means that if you want to go to all the practices, you will need to do the school work you miss at another time and stay at your grade level." Is a different message than "You can miss school for gymnastics, no matter the consequences, because gymnastics is more important." Problem solving this issue with your child is a learning exercise in itself.
My priority for my kids has always been learning, not "education." To me this means things like learning to think critically, to problem solve, to reason, to apply logic to an issue, as well as developing the ability to function within and be an asset to a group. Developing resiliency and what is currently called "growth mindset" but used to be known as a "can do attitude" is also important. It is also necessary, of course, to acquire facts and skills, but that is not all there is. Learning on this wider scale potentially happens in any environment, so a decision like this depends on the child's abilities and attitude as well as what kind of learning is happening in the various environments (gym, school and home.)
I want to be clear I do not think there is any right or wrong decision here. I am just offering the other perspective. It might well be much better for your child to miss gym that day rather than school. I have no idea how much missing this one day of practice a week is going to matter to your child's gymnastics. It may be that this missed day is not much of an issue at all. Another possible solution to consider as you problem solve together might be for your daughter to think how she can work extra hard on the days she is in the gym to make up for the missed practice, or maybe there is an open gym she can attend on a weekend, or whatever. It might be smart to discuss these options, as it teaches that whatever decision is made, there are trade offs and compromises. This is a good life lesson.