gloominess vs melancholy

gloominess and melancholy both are nouns.

Word	Noun	Adjective	Verb	Adverb
gloominess	Yes	No	No	No
melancholy	Yes	Yes	No	No

As nouns, melancholy is a hypernym of gloominess; that is, melancholy is a word with a broader meaning than gloominess:

Other hypernyms of gloominess include apprehension, apprehensiveness, dread.

gloominess (noun)	melancholy (noun)
the quality of excessive mournfulness and uncheerfulness	a humor that was once believed to be secreted by the kidneys or spleen and to cause sadness and melancholy
The property of having a frowning or scowling appearance.	a feeling of thoughtful sadness
a feeling of melancholy apprehension	a constitutional tendency to be gloomy and depressed
an atmosphere of depression and melancholy

gloominess (adjective)	melancholy (adjective)
	grave or even gloomy in character
	characterized by or causing or expressing sadness