Yours, Etc.: Origins and Uses of 8 Sign-Offs
mid-15c., "Roman swordsman," from Latin gladiator, literally "swordsman," from gladius "sword," probably from Gaulish (cf. 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]