The Japanese Nomenclature for Gladiator consisted of 士 which translates to “User” or “Specialist”, 術 which translates to “Art”, as well as the Weapon itself 剣 “Sword”. This became 剣術士 (ken-jutsu-shi) which literally means “Sword Art User” or “User of the Sword Arts.” But simply calling them “Sword User” would be too bland to most English speakers so they added flair. 剣 translates into Sword in English. Sword in Latin is Gladius. Add the latin -ātor, meaning “User of”, and we are left with Gladiator. Gladiator is certainly a more palatable name than Sword User.
Other classes follow the same pattern:
剣術士 → Sword User|Sword User→Gladius (Latin: Sword)+ -ātor (Latin: Agent) → Gladiator
格闘士 → Hand-to-Hand User|Fist User → Pugil (Latin: Fist) + -ist (Latin: Practitioner) → Pugilist
弓術士 → Bow User|Bow User → Arcus (Latin: Bow) + -er (Defines a User from his Tool) → Archer
槍術士 → Lance User|Lance User → Lancea (Latin: Lance) + er (Defines a User from his Tool) → Lancer