This is fun - I translated this one as 'gladiator should fight' because I got 'gladiatores pugnare debent' wrong when I translated it as 'the gladiators should fight' not 'gladiators should fight' (correct version, apparently). Trying to out-think this clearly a bad idea. :)
English "wants" an article depending on the context, the nature of the noun, and whether the noun is singular or plutal. So, the other sentence with "gladiatores" would correctly be either "the gladiators" or just "gladiators." In this case, unless "Gladiator" was somebody's proper name, in English we would want "a" or "the," just as we wouldn't say "Pencil should write" in English. Which pencil? A pencil or the pencil. But "pencils should write" is OK. There's probably some interesting linguistic reason for this that I don't know, but for my native English speaker ears, "gladiator should fight" is like fingernails on a chalkboard.