My child is in Australia, and is an IDP gymnast (elite path). She will soon be 9. Next year because of the IDP level that she will then be at, the gym will expect her to miss school two afternoons per week. There are girls at her gym who have been doing this since they were 7, due to the fact that they reached the appropriate IDP levels at an earlier age.
I believe that my daughter will also start late on a couple of days per week because of early morning training sessions. The gym organises training in this way so that the elite girls have uninterrupted access to the equipment, rather than having to fight the national level and recreational gymnasts for time on the equipment.
I think our gym works it so that there are three early afternoon training sessions, and you choose the two that will be least disruptive to your child's schooling. They encourage the child to be at gym for early afternoon training on the afternoon that the school has sport (within school hours), if sport is timetabled that way at the school that your child attends.
This is the norm here for IDP gymnasts who reach a certain level. I've already flagged it with our school Principal that this will happen. The Principal was extremely supportive (actually she was a little star-struck!), and said that they will organise for the correct Leave of Absence provisions for her when the time comes. In the same way that children who are in the entertainment industry can get Leave of Absence, so can children doing elite sports.
So while it doesn't seem to be usual in the US, it is quite common here in Australia for gymnasts who are on the track towards the Olympics. High performance gyms often work hand in hand with partner schools, which the girls are encouraged to attend.
You just have to make the best of it that you can. I completely understand the gym's reasoning for it. Sure, I'd prefer that my daughter was in school for all the school hours, but that's not the way it works here.
The word is that girls who have represented Australia in gymnastics tend to be highly intelligent, good students, who go on to excellent university courses (requiring high marks) once they have finished school. If they were missing too much school and their school education was faltering, their continuing education at such a high level would not be possible. The fact that they are managing it seems to indicate that it is possible.
Good luck with your decision making.