gentlewoman vs adult female

gentlewoman and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
gentlewoman	Yes	No	No	No
adult female	Yes	No	No	No

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

Other hypernyms of gentlewoman include woman.

gentlewoman (noun)	adult female (noun)
a woman of refinement	an adult female person (as opposed to a man)