Doctor Merkury has a 54mm gladiator arena floorplan for you to print-and-play.
Standard Wargame Rules For Ancient and Medieval Warfare is an alternative to DBM with the same basing. The authors write:
The designer team includes both extremes of wargamers: dedicated tournament players as well as those who prefer historical re-fights. These rules aim to satisfy these both sides of the wargaming hobby. We wanted to write a set of rules that allows playing realistic and interesting re-fights of historical battles. Furthermore, we have spent much time to make the rules as exact as possible, so that they are also suitable for tournament play. Once both players know the rules, there should be no need for an umpire or for long rules discussions during the game.
One aim was also to design a rule set that is “easy to learn but difficult to master”. This means that you can learn the rules relatively quickly, but will probably need several games, before finding the optimal tactic for your army. Also, there are no “super troops”. Each troop type has certain strengths and weaknesses that you will need to learn.
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.
10,000 In My Pocket is a free solo print-and-play game about the conflict between Persia and Sparta.
A Great Victory is a set of free wargames rules for miniature wargames set in the pregunpowder era.