Duels of Cartisora is a pit fighting game played with a regular deck of cards. I rather like the mechanism used to determine the outcomes.
The Games: Blood and Spectacle are a set of free wargames rules for gladiator games.
Doctor Merkury has a 54mm gladiator arena floorplan for you to print-and-play.
Munera Sine Missione, Exagoni Libero is a set of free wargames rules for gladiator combat. It is based Munera Sine Missione, but unlike that set doesn’t use a hex mat.
Here are a set of houserules for the Warhammer Gladiator game.