fighter vs gladiator

fighter and gladiator both are nouns.

Word	Noun	Adjective	Verb	Adverb
fighter	Yes	No	No	No
gladiator	Yes	No	No	No

As nouns, gladiator is a hyponym of fighter; that is, gladiator is a word with a more specific, narrower meaning than fighter:

fighter: someone who fights (or is fighting)
gladiator: (ancient Rome) a professional combatant or a captive who entertained the public by engaging in mortal combat

fighter (noun)	gladiator (noun)
a high-speed military or naval airplane designed to destroy enemy aircraft in the air	(ancient Rome) a professional combatant or a captive who entertained the public by engaging in mortal combat
someone who fights (or is fighting)	a professional boxer
someone who fights for a cause