Rome has a rather complicated relationship with its circuses -- it was a religious ceremony, entertainment for the masses, and uncouth blood sport unfit for Roman sensibilities all at the same time. I took a course with Jeff Stevens during my time at UCLA on ancient Greece and Roman sports, who worked as a historical consultant for the show Spartacus which I watched and enjoyed myself.

First, I would absolutely agree that the select few "elite" gladiators enjoyed high standards of living in their positions despite officially being slaves. A number of successful gladiators were granted land after their tenure as a reward for their service, much like retired legionnaires would get as their retirement pension. However, the whole gladiatorial games were theatrical in many respects, and the "fan-favorites" would pretty much be guaranteed wins after wins because it was a sport that was built on the personality of each fighter. In a way, it's pretty comparable to the star system of Hollywood I suppose.

To answer your actual question, I don't know of any exact examples. By the time the gladiators won their freedom, they were old and very popular in the city, basically not in any position to join the legions. However, there have been cases in which generals of Rome had recruited slaves out of desperation. The following passage is from Alison Futrell's The Roman Games, which one of the texts I used for the course I mentioned earlier:

"Gladiators were perceived as more of a security risk in the last years of the Republic, allegedly recruited to fight for a number of ambitious and radical politicians. "Escaped" gladiators had a very different experience in 31-30 CE, when a troupe of gladiators drafted to the cause of Antony were cut adrift in the days following Antony's catastrophic loss at the Battle of Actium. The presence of gladiators enrolled in the Roman legions was a sign of confused and desperate times; normally, Rome's generals scorned such in fames people as unworthy of the honor of fighting for Rome."

Once again, I'd say gladiators serving alongside legions offended the "Roman sensibilities" as they were still slaves without full rights. Futrell cites an account written by Cassius Dio, which basically says while everyone else refused to lend a hand to the defeated Antony, the gladiators willingly filled the ranks. In this case, they're depicted as the models of loyalty (to the late republic I think it's saying) but in any other case, these peoples were unworthy of fighting for Rome. If anything, they certainly were used for military purposes, but based on Futrell, I think it's suggesting that they were mercenaries within Rome during times of internal crisis rather than mercenaries fighting Rome's external enemies. Hope this helps.

In the Roman empire there are basically two kind of slaves: those out of the "barbaric" regions and those out of "civilized" regions. Slaves who fought as gladiators and those who worked in the fields and mines were of the former category. They were often captured slaves and their descendants, and they were barely regarded as human beings.

Opposite to them, the latter category included a lot of people who were not captured, but simply amassed a high debt and had to sell themselves (or be sold by their parents) into slavery. These were for example Greeks or even Roman farmers, sometimes quite educated, and they belonged to the household of their masters (the farm slaves didn't). They were under the serious protection of the pater familias and often worked the intellectual jobs: as teachers, aides, secretaries and businessmen. For the Roman nobility it was considered bad taste to engage in trade or commerce, so they let their slaves do it for them. It was very common for some of them to become rich and eventually get freed. Those freeman were an important social class in the Roman Empire and their children were proper Roman citizens. Freeing household slaves was very common, at the latest of the death of their owner. They still remained a sort of client of their former master, however.

These are of course only simple categories. Gladiators were somewhere in between them. There is a story of a Gladiator named Flamma who supposedly chose not to retire and keep on fighting, but that seems to be an exception. As freed Gladiators were not Roman citizens, they could not legally join the Roman legions, only the auxilliary troops. I don't know if this has ever happened, but then again, there are only stories about very famous Gladiators around. What happened to the supposedly larger number of not quite as famous, but retired Gladiators? Hard to say. Gladiators do appear in the entourage of emperors (Nero for example), and possibly they hired themselves out for various jobs.