often vs a great deal

often and a great deal both are adverbs.

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

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

Other synonyms of often include much.

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