gentle vs calm down

gentle is an adjective but calm down is not an adjective.

Word	Noun	Adjective	Verb	Adverb
gentle	No	Yes	Yes	No
calm down	No	No	Yes	No

As verbs, calm down is a hypernym of gentle; that is, calm down is a word with a broader meaning than gentle:

Other hypernyms of gentle include calm, lull, quiet, quieten, still, tranquilize, tranquillise, tranquillize.

gentle (adjective)	calm down (adjective)
marked by moderate steepness
having little impact
soft and mild; not harsh or stern or severe
belonging to or characteristic of the nobility or aristocracy
quiet and soothing
easily handled or managed
having or showing a kindly or tender nature

gentle (verb)	calm down (verb)
stroke soothingly	become quiet or less intensive
cause to be more favorably inclined; gain the good will of	make calm or still
give a title to someone; make someone a member of the nobility	become quiet or calm, especially after a state of agitation