mammoth vs gigantic

mammoth is a noun but gigantic is not a noun.

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

As adjectives, mammoth and gigantic are synonyms defined as:

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

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

mammoth (adjective)	gigantic (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