That's not the way it worked. Gladiators proper were well-trained, extremely expensive professionals and only fought with other gladiators. They were often slaves (though with extremely privileged status, and only a couple fights was usually enough for them to buy their freedom and retire comfortably), though they could also be free men who had volunteered for the promise of massive riches, or disgraced free men (I'm a bit unclear on what that means exactly, because Cicero doesn't explain himself). Usually, though, gladiatorial fights were not between criminals, and we have none of this Ridley Scott nonsense with chained slaves rushing up against a couple of trained fighters, or a trained gladiator fighting wild beasts. Beast hunts were carried out by trained beast-fighters, not gladiators, and the two did not mix. Criminals sentenced to death in the arena, at least when Seneca was writing (and this form of execution might very well be a feature of Nero's weirdness), fought during the midday games in a big melee, chained to each other. They did not fight trained gladiators, and though one pair of criminals would survive (or one and the corpse chained to him) he would just be thrown back into the arena the next day

EDIT: Okeedoke, so with regards to the length of gladiatorial careers, their life expectancy and other questions that have popped up:

Most of our graffiti on gladiators note that successful gladiators would have careers of only a few matches, although later on we do appear to have more detailed records of gladiatorial careers (but I don't actually study gladiators at all, so it'll be a bit difficult for me to find those). There appears to be a good deal of debate on just how much a gladiator could earn--we hear of gladiators being awarded huge amounts of money or important positions and honors (Suetonius reports that Tiberius offered some retired gladiators 100,000 sesterces to go back into the arena, and that Nero gave a murmillo named Spiculus "triumphalium virorum patrimoniis aedibusque," the estates and inheritance of triumphators, and Cassius Dio claims that Antony's bodyguard was made up of successful gladiators, a practice that was not unknown--c.f. Pro Milone, the OCD mentions that it appears to have been a common practice at the end of the Republic), but these are clearly outliers. Still, a successful gladiator could hope to earn a fair amount--enough to earn his freedom surely, as is evidenced by all the tombstones of former gladiators that say that they bought their freedom and retired. Gladiators were paid their prize money in cash after a fight (and apparently most then split it with their trainers)--Suetonius relates that Claudius used to count the prize money aloud as he handed it to the victor (at least in this case the prize money must have been somewhat substantial, as Suetonius says that the prize money was paid in aurei, gold coins, which were only used for very large sums that could not easily be dealt in denarii or the more usual sesterces). This I think matches up well with what we know of other professional athletes, particularly chariot-racers, who as far as we are aware could amass rather large fortunes if they were good. Now, less successful gladiators probably had very different stories, and I think this comment from the other day is a good example of that--the inscriptions he cites contain careers much longer than many fighters (graffiti from Pompeii tend to show something like 3-5 fights, but remember these are graffiti, so they tend to be celebrating the careers of star gladiators, not the average). His comment that this also only applies to professionals is very important, and I'm excited to take a look at the books he's listed, as they seem to stress the difference between professional fighters and others. Gladiators were a class apart from normal contemned criminals, but they appear to have often been prisoners-of-war--Spartacus may have been one (although the tradition surrounding Spartacus' origins is very confused), and as I recall Josephus says that many of the captives taken in the Jewish Revolt were sent to the arena either as untrained noxii (criminals) or trained fighters (but I never paid any attention to Josephus so I'd have to check). It seems likely that prisoners may have been considered a separate, lower-class form of gladiators as compared to slaves bought for the purpose of gladiatorial competition or volunteers, and I hope the books cited in the post I linked to have something to say about it, because I'm rather interested in what we know about that. As a final note on this, Tacitus says that the free volunteers who became gladiators often did so out of the promise of wealth:

Priores id principes pecunia et saepius vi perpulerant, ac pleraque municipia et coloniae aemulabantur corruptissimum quemque adulescentium pretio inlicere.

Previous emperors [This is in Tacitus' passage on Vitellius, so those before 69] had compelled them [the equestrians] with money and more often by force [Tacitus is speaking of limits that were made on the ability of Roman equestrians to volunteer as gladiators], and very many towns and colonies would emulate the emperors in attracting the worst of their young men with money [inlicere can have a number of meanings, including "seduce," or "mislead," and pretium can mean either just the money handed over in a sale or the prize money of a competition, among a number of meanings--I'd have to do some thinking to figure out which one Tacitus means here, although I'm inclined to say the former]

Regarding the life-span of fighters, which someone asked about, I don't think we can say. Both the short careers of successful gladiators and the longish careers of the ones in /u/ChiefoftheCharles's comment would indicate that gladiators survived a long time--in the former case they could buy their freedom quickly, and in the latter they obviously survived long enough to fight lots of times (although I think the 34-fight career of on gladiator he mentioned must be something of an outlier! That's a crapload of fights!). Plus there's things like the raising of a finger to surrender, which would end the fight according to Martial. But we have no statistics on things like life expectancy for soldiers or gladiators, and honestly soldiers appear to have pretty high life expectancies. In the Principate legions tended to sit at their camps and engage in raiding or defensive actions, rather than going on active campaign, and casualties appear to usually have been light. In the late Republic a soldier in a single army usually was only under arms during the term of his general's magistracy--most armies were led by promagistrate governors, whose usual term was five years and who were supposed to disband their armies after their mandate ended (although some, like Pompey, refused).

DOUBLE EDIT: I checked the OCD and I realized I made a slight error. The OCD claims that criminals, of the type that Seneca speaks of did sometimes fight beasts. So there you go. While I'm on the subject, I should probably clarify a bit about the criminals. Seneca describes criminals condemned to death being chained in pairs, one with a sword and the other with a shield. Neither was armored, and they just fought other pairs in this big melee until basically everyone died. Seneca objects to the fact that since they're unarmored and untrained it's just butchery without any honor or anything at all, since every thrust of the sword lands a fatal hit (oddly, Cicero thinks that condemned criminals in the arena--the treatment of the condemned in the arena may have been different in Cicero's time, with criminals fighting in pairs like regular gladiators, but we have very little evidence regarding the specifics of gladiatorial contests before around Augustus--is extremely honorable, since the condemned does not shrink from the blow, but takes it nobly, but then again he's probably describing a different custom than Seneca is). Pretty much everything said about gladiators in this post does not apply to these poor guys. Gladiators could be captives, usually captured in war, or they could be slaves specifically bought for fighting (or free volunteers). But not criminals, at least not as far as we know. Those were not called gladiators, though they did fight