In the Scence where the sister speaks to Commudos, after he finds out that Maximus is still alive, there is a rectangular column in the background. It has reveals on each corner!
This type of column didn't exist in the Roman empire! It was a variation started in the MAnnerist period after the reniassance. (over a 1000years later)
If you think about it, most people who saw Gladiator were not historians or had that kind of architectural background knowledge. Not to mention that not every movie can be perfectly historical accurate. You are very keen to pick that up, I must admit, I would never have figured any of that out.
This is true.. but what you have to realize is that many cinematographers sometimes add these structural designs BECAUSE people lack certain knowledge. They see the reveals and think "Ok, Roman Empire." So, instead of being historically accurate, they go with what the public believes is correct so that they can paint a better picture and make a better atmosphere for the movie.