a great deal vs a good deal

a great deal and a good deal both are adverbs.

Word	Noun	Adjective	Verb	Adverb
a great deal	No	No	No	Yes
a good deal	No	No	No	Yes

As adverbs, a great deal and a good deal are synonyms defined as:

Other synonyms of a great deal include a lot, lots.

a great deal (adverb)	a good deal (adverb)
to a very great degree or extent	to a very great degree or extent
frequently or in great quantities