viol vs viola da gamba

viol and viola da gamba both are nouns.

Word	Noun	Adjective	Verb	Adverb
viol	Yes	No	No	No
viola da gamba	Yes	No	No	No

As nouns, viola da gamba is a hyponym of viol; that is, viola da gamba is a word with a more specific, narrower meaning than viol:

viol: any of a family of bowed stringed instruments that preceded the violin family
viola da gamba: viol that is the bass member of the viol family with approximately the range of the cello

Other hyponyms of viol include viola da braccio, bass viol, gamba, viola d'amore.

viol (noun)	viola da gamba (noun)
any of a family of bowed stringed instruments that preceded the violin family	viol that is the bass member of the viol family with approximately the range of the cello