Hell vs Gehenna

Hell and Gehenna both are nouns.

Word	Noun	Adjective	Verb	Adverb
Hell	Yes	No	No	No
Gehenna	Yes	No	No	No

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

Hell: (Christianity) the abode of Satan and the forces of evil; where sinners suffer eternal punishment
Gehenna: a place where the wicked are punished after death

Other hyponyms of Hell include Tartarus, hellfire, red region.

Hell (noun)	Gehenna (noun)
(Christianity) the abode of Satan and the forces of evil; where sinners suffer eternal punishment	a place where the wicked are punished after death
(religion) the world of the dead