Anything he wants! :^)
A diet of cereals makes sense, since gladiators were slaves, and slaves were generally fed cereals. I'd never given it any thought, either. :^) "Caesar! Who made the salad?" "Me, and Seven Seas!"
But seriously, way back when, we all learned about who ate what, in Ancient Rome, in first year Latin class...which I still remember. And Apius, or some such Ancient Roman wrote a cooking book, which we all had to read in Latin and translate. I guess that's why I remember it so well. :-)