It was mostly vegetarian and apparently not very appetizing.

Researchers did an isotope anaylsis of gladiator bones found in a grave at Ephesus (there was a tablet marking the gravesite as a gladiator grave, hence). The Medical University of Vienna team found their diet was heavy on barley, beans, and legumes.

Pliny refers to gladiators as hordearii or “barley eaters," and other authors refer to gladiators eating mash, so the find at Ephesus matches textual evidence for gladiator diets.

The Vienna team also suggested that the gladiators drank a mixture of ashes and vinegar as a foul-tasting nutritional supplement.

It's a very high-energy, high-protein diet. It's perfect for the premier professional athletes of the era, who spent hours every day in training. Being vegetarian, it's also cheaper, hence why they probably didn't eat much meat.

(As a side note, one of the authors of the Vienna study has suggested that the gladiators intentionally ate a high-carb diet to put on weight...as a kind of protective fat layer. See what I've written below for why this is almost certainly bullshit).

https://www.reddit.com/r/AskHistorians/comments/a6fobc/comment/ebw4h6r