gloom vs melancholy

gloom and melancholy both are nouns.

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

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

Other hypernyms of gloom include apprehension, apprehensiveness, dread.

gloom (noun)	melancholy (noun)
a feeling of melancholy apprehension	a humor that was once believed to be secreted by the kidneys or spleen and to cause sadness and melancholy
a state of partial or total darkness	a feeling of thoughtful sadness
an atmosphere of depression and melancholy	a constitutional tendency to be gloomy and depressed

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