Bangs should be just fine as long as they don't cover her eyes, bother her, or look messy. If they were the longer side swept bangs that are popular with older girls, I think they'd need to be clipped back, if only for the comfort of the gymnast. Most things aren't really "banned" so much as there is an expectation you will conform to standards of decency and good taste (I'm pretty sure that's about the exact way they word it).