Ex-Gladiators as Centurions

[Gaius Colletti]

"any gladiator (minus volunteers perhaps, and possibly criminals) who left the arena would become freedmen"

Where do you get this idea from?

I got that from my understanding that Gladiators were slaves, and slaves who were set free became "freedman". I would assume since many if not most gladiators were technically slaves that they too would become freedmen if they were set free