plenty vs great deal

plenty and great deal both are nouns.

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

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

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

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

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