For most gladiators they didn't have a choice in what to wear. They were
either slaves sold to the ludus, captured criminals, or men captured during war time. Very few free men/citizens signed
up to be gladiators but even then they didn't have much say so in what they wore.
In almost every depiction of gladiators, they are wearing a subligaculum or loincloth.
Now we believe this comes from the ancient Greeks that trained in the nude so that no clothing could hinder their movements
but because the Roman moral code did not allow for public nudity they wore the subligaculum.
To hold up the subligaculum they wore a balteus. This belt was two or three inches
wide and typically made of of bronze with a leather lining. We believe the belt came from the Samnites, an old enemy
of Rome, that when the boys became men they wore a bronze belt to distingish themselves as a man.