a great deal vs often

a great deal and often both are adverbs.

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

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

Other synonyms of a great deal include much.

a great deal (adverb)	often (adverb)
to a very great degree or extent	many times at short intervals
frequently or in great quantities	frequently or in great quantities
	in many cases or instances