giant vs gargantuan

giant is a noun but gargantuan is not a noun.

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

As adjectives, giant and gargantuan are synonyms defined as:

Other synonyms of giant include elephantine, jumbo.

giant (noun)	gargantuan (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)	gargantuan (adjective)
of great mass; huge and bulky	of great mass; huge and bulky