There is precedent for "Free" gladiators in the city, though most of the time they do not compete with the real Gladiators. Its a bit like having the Chief of Police fight a WWE Wrestler (if wrestling were real). They both may be physically fit, but one is professionally trained in what he does, the other is an amateur.
You can fight in the Arena, and you can fight some gith gladiators, or sentenced-to-death elves and other criminals. I don't know how the Gladiator owners would feel about "Joe Schmoe" fighting their prized Champion Gladiator though.