Here is the quote from The Twelve Caesars, " In the gladiatorial contest in the Forum Furius Leptinus, a man of praetorian stock, and Quintus Calpenus, a former senator and pleader at the bar, fought to a finish".
Senators fighting as gladiators was not unheard of in the republican period. Balsdon and Lintott attribute this to the 'macabre glamour', even the sexiness, of the vocation.
In the first century AD, legislation under Augustus and Tiberius forbade members of the senatorial and equestrian orders from fighting as gladiators.
But your initial instinct that gladiators were generally slaves, freedmen, prisoners of war, and criminals is correct.
Sources:
Usually Many gladiators indeed were slaves, but it became more and more popular to condemn somebody to fight during circus games in the Late Republic and later.
One possibility was to sentence delinquents (especially political enemies) to fight unarmed against beasts (damnatio ad bestias) or trained gladiators (damnatio ad ferrum/damnatio ad gladium) which more or less equals a sentence of death.
The damnatio ad ludum gladiatorum forced the condemned to fight as a gladiator. Being considered a gladiator he had to follow the games' rules and could in case of defeating his opponent(s) survive, maybe even live a free man's life after several victories.
In this case, Furius Leptinus and Quintus Calpenus fought for Pompey and had to face Caesar's sentence. I'm not sure about this, but as far as i know, this is the first case of a former senator being condemned this way. Suetonius probably tries to put some special stress on how disrespectfully Caesar treats his enemies in general and social elites like the senate.