Here is what the USAG Rules and Policies state regarding appearance(page 5):
9. Be well groomed in her appearance:
a. Clean attire.
b. Hair secured away from the face so as to not obscure her vision of the apparatus.
c. No jewelry, with the exception of one pair of stud earrings (one in each ear). All other piercing should be
REMOVED, not just covered with tape or Band-Aids.
Of course, as others have stated, your gym may have it's own rules as well.