Final answer:
The adjective 'gargantuan' comes from the giant king Gargantua in Rabelais' work, which denotes something of immense size or volume.
Step-by-step explanation:
The adjective that comes from Rabelais' giant king is Gargantuan. François Rabelais was a French Renaissance writer known for his work 'Gargantua and Pantagruel', which is a series of books that tell the story of two giants; one being Gargantua. The term gargantuan has since been adopted into the English language to describe something that is extremely large or gigantic, mirroring the immense size and appetite of the character Gargantua.