calm down vs gentle

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

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

As verbs, gentle is a hyponym of calm down; that is, gentle is a word with a more specific, narrower meaning than calm down:

Other hyponyms of calm down include lull, compose, appease, assuage, conciliate, gruntle, lenify, mollify, pacify, placate.

calm down (adjective)	gentle (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

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