gentleness vs manner

gentleness and manner both are nouns.

Word	Noun	Adjective	Verb	Adverb
gentleness	Yes	No	No	No
manner	Yes	No	No	No

As nouns, manner is a hypernym of gentleness; that is, manner is a word with a broader meaning than gentleness:

Other hypernyms of gentleness include personal manner.

gentleness (noun)	manner (noun)
acting in a manner that is gentle and mild and even-tempered	a way of acting or behaving
the property possessed by a slope that is very gradual	how something is done or how it happens
	a kind