We tell our compulsory girls and new compulsory parents "no makeup, no nail polish, small stud earrings okay, hair pulled back snuggly and neatly." It they put in glitter hair spray, that's fine. No rule against it but we also don't encourage it (try vacuuming that up!). Our justification is that they should be focusing on their athletic performance, not getting all "dressed up" or "dolled up." Athletes, not beauty pageant contestants.
As gymnasts get to be teens, many wear "natural" makeup to school and then practice or at meets. That's fine.