Well, as I have said many times before in the past, Gladius was a very good Gladiator style RPG game. In fact, I liked the game so much that my username is taken from one of the Gladiator classes you can play in the game, "Minotaur". My only problem with the game was the lack of a sequel, and unfortunately LucasArts is to blame for that.. anyways, Gladius (since the game came out a while back) can be bought pretty cheep in a Gamestop for either the Gamecube, PS2, or Xbox.
Shadow of Rome was a nice Gladiator RPG game too, its more violent and gorier for people who are looking for the full Gladiator in a arena experience. The graphics were nice too for being an older game and the price is still pretty affordable (aprox. $10).
If you want an online gladiator RPG game, Gladius II is a good game. This isn't the sequel to the LucasArts Gladius but it is still a nice online MMORPG game for what it is. You can sign up for the game service here:
www.arenasofglory.com