giant vs gigantic

giant is a noun but gigantic is not a noun.

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

giant (noun)	gigantic (noun)
any creature of exceptional size
an unusually large enterprise
a very bright star of large diameter and low density (relative to the Sun)
someone that is abnormally large and powerful
a person of exceptional importance and reputation
an imaginary figure of superhuman size and strength; appears in folklore and fairy tales
a very large person; impressive in size or qualities

giant (adjective)	gigantic (adjective)
of great mass; huge and bulky	so exceedingly large or extensive as to suggest a giant or mammoth