For a limited time, the Habet, Hoc Habet! Rules from Scale Creep Miniatures is available on a pay-what-you-want basis. These are designed for fast combat among Roman Gladiators, including campaign and gambling rules.
Roar of the Crowd is a set of free rules for playing science fiction, fantasy or historical gladiator games.
Andy Watkins has one page set of rules for games involving Roman gladiators. You can use the fine Wargames Foundry 28mm Gladiators to fill your arena.
Familia Gladiatoria is a set of free wargames rules for gladiator campaigns.
Thumbs Up, Thumbs Down is a set of free wargames rules for gladiatorial combat. The rules are designed for up to ten players.