a lot vs a great deal

a lot and a great deal both are adverbs.

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

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

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

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