gentleman vs adult male

gentleman and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
gentleman	Yes	No	No	No
adult male	Yes	No	No	No

As nouns, adult male is a hypernym of gentleman; that is, adult male is a word with a broader meaning than gentleman:

Other hypernyms of gentleman include man.

gentleman (noun)	adult male (noun)
a man of refinement	an adult person who is male (as opposed to a woman)
a manservant who acts as a personal attendant to his employer