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.
Heresy Miniatures offers a set of free miniatures rules called Deathball. There’s also an arena for you to download.
In Deathball, the authors write:
The team’s objectives tend to revolve around the placing of heads and the inflated organs of sacrificial victims into stone goals or onto mighty altars, while at the same time killing and maiming as many of the opposing dark lord’s fighters as possible. In Deathball, deaths are frequent and spectacular: the ‘ball is always the secondary consideration. It is the roar of the crowd, that greets the players with howls of lust and fury as gold changes hands; the laughter of the Dark Lords as another creature is slaughtered; the thrill and relief of surviving; this is what Deathball really is!