gigantic vs mammoth

gigantic is not a noun while mammoth is a noun.

Word	Noun	Adjective	Verb	Adverb
gigantic	No	Yes	No	No
mammoth	Yes	Yes	No	No

As adjectives, gigantic and mammoth are synonyms defined as:

gigantic and mammoth: so exceedingly large or extensive as to suggest a giant or mammoth

gigantic (noun)	mammoth (noun)
	any of numerous extinct elephants widely distributed in the Pleistocene; extremely large with hairy coats and long upcurved tusks

gigantic (adjective)	mammoth (adjective)
so exceedingly large or extensive as to suggest a giant or mammoth	so exceedingly large or extensive as to suggest a giant or mammoth