We just played a game of Fistful of Kung Fu. Although we were using sci-fi miniatures, I kept an eye out for its effectiveness as a gladiatorial game. I reckon it would work really well. It is - by design - pulpy and larger than life, but that probably suits an arena set-up well, as it's theatrical and entertaining.
One of the key differences between the game and others in the Ganesha/SoBH stable is that you get an even wider variety of potential combat outcomes. The core SoBH rules are pretty good for this already, with their knock-downs and push-backs, but AFFoKF has wounds, disarming (in various forms), grappling, distraction, knock-outs, humbling, stunning, staggering, laming, blinding and activation of scenery.
The emphasis on dropped weapons is particularly appropriate for gladiators, I reckon, and there's an Entangle rule that's perfect for retiarii.
And again, the various special rules are highly intuitive. You'd probably check a few of them the first time you play and then never need to look them up again; that's certainly been our experience with SoBH-type rules.