dress up vs garment

dress up is not a noun while garment is a noun.

Word	Noun	Adjective	Verb	Adverb
dress up	No	No	Yes	No
garment	Yes	No	Yes	No

As verbs, garment is a hypernym of dress up; that is, garment is a word with a broader meaning than dress up:

Other hypernyms of dress up include apparel, clothe, dress, enclothe, fit out, garb, habilitate, raiment, tog.

dress up (noun)	garment (noun)
	an article of clothing

dress up (verb)	garment (verb)
put on special clothes to appear particularly appealing and attractive	provide with clothes or put clothes on
dress in a costume
dress in a certain manner
make something appear superficially attractive
put a caparison on