I second the above comment. That’s so far from appropriate that it would make me worry about allowing my daughter to spend time in that environment. The word fat should never be uttered by an adult in a gym regardless of the circumstances, and no non-medical person should be talking to children about puberty blockers.
Definitely follow up, as the coach who said that needs some training so it doesn’t happen again. What are the USAG rules around coaches making that sort of comment? Do they make training resources available for gyms?
In Australia, you don’t get accredited to coach or judge until you’ve done the body image training which makes it clear that it is totally against GA rules for coaches, gym owners or judges to comment on weight, height, body shape, development, etc except for under highly specific circumstances (like sorting athletes into pairs/trios for acro gymnastics, and even that has a fairly limited range of things you can say).