mid-15c., "Roman swordsman," from Latin gladiator (fem. gladiatrix) "fighter in the public games; swordsman," from gladius "sword" (there is no verb *gladiare), which probably is from Gaulish (compare Welsh cleddyf, Cornish clethe, Breton kleze "sword;" see claymore). Old Irish claideb is from Welsh.
The close connection with Celtic words for 'sword', together with the imperfect match of initial consonants, and the semantic field of weaponry, suggests that Latin borrowed a form *gladio- or *kladio- (a hypothetical variant of attested British Celtic *kladimo- 'sword') from [Proto-Celtic] or from a third language. [de Vaan]