The film is called, simply, Gladiator. Not The Gladiator, or A Gladiator, or Gladiators.
And this is totally apt, because it draws attention to Maximus's story. This is his movie, after all, and all attention must be directed to him.
But there's another layer to consider. When Maximus and Commodus confront each other in the arena in the middle of the film, Maximus refuses to tell him his name. He simply says,
MAXIMUS: My name is…gladiator.
This makes it clear: this film is an exploration of Maximus's assumed identity as an (initially) anonymous gladiator. This isn't General Maximus's story: this is the story of Maximus after he falls from grace.