SPOILER ALERT:

Gladiator is literally a remake of a 1964 epic called The Fall of the Roman Empire.

Anyway, both movies take lots of liberties with the facts. The central plot is one example. In both movies, Commodus has a rivalry with Livius/Maximus and ends up duelling him to the death (this didn't happen in real life). Also, the rival has a romance with Lucilla (Commodus' sister) which is also fiction.

Nevertheless, both movies are worth a watch if you're not that interested about historical accuracy and just want a decent movie.