nightwear vs gown

nightwear and gown both are nouns.

Word	Noun	Adjective	Verb	Adverb
nightwear	Yes	No	No	No
gown	Yes	No	Yes	No

As nouns, gown is a hyponym of nightwear; that is, gown is a word with a more specific, narrower meaning than nightwear:

nightwear: garments designed to be worn in bed
gown: lingerie consisting of a loose dress designed to be worn in bed by women

Other hyponyms of nightwear include night-robe, nightdress, nightgown, nightie, nightshirt, jammies, pajama, pj's, pyjama.

nightwear (noun)	gown (noun)
garments designed to be worn in bed	a woman's dress, usually with a close-fitting bodice and a long flared skirt, often worn on formal occasions
	outerwear consisting of a long flowing garment used for official or ceremonial occasions
	protective garment worn by surgeons during operations
	lingerie consisting of a loose dress designed to be worn in bed by women
	the members of a university as distinguished from the other residents of the town in which the university is located

nightwear (verb)	gown (verb)
	dress in a gown