great deal vs plenty

great deal and plenty both are nouns.

Word	Noun	Adjective	Verb	Adverb
great deal	Yes	No	No	No
plenty	Yes	No	No	Yes

As nouns, great deal and plenty are synonyms defined as:

great deal and plenty: (often followed by `of') a large number or amount or extent

great deal (noun)	plenty (noun)
(often followed by `of') a large number or amount or extent	a full supply
	(often followed by `of') a large number or amount or extent

great deal (adverb)	plenty (adverb)
	as much as necessary