great deal vs mountain

great deal and mountain both are nouns.

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

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

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

great deal (noun)	mountain (noun)
(often followed by `of') a large number or amount or extent	a land mass that projects well above its surroundings; higher than a hill
	(often followed by `of') a large number or amount or extent