down in the mouth vs gloomy

down in the mouth and gloomy both are adjectives.

Word	Noun	Adjective	Verb	Adverb
down in the mouth	No	Yes	No	No
gloomy	No	Yes	No	No

As adjectives, down in the mouth and gloomy are synonyms defined as:

Other synonyms of down in the mouth include blue, depressed, dispirited, down, downcast, downhearted, grim, low-spirited, low.

down in the mouth (adjective)	gloomy (adjective)
filled with melancholy and despondency	depressingly dark
	filled with melancholy and despondency
	causing dejection