'gargantuan' originates from French, specifically the name 'Gargantua' (the giant character created by François Rabelais in the 16th century), and from French 'gargantuesque' referring to things of Gargantua's size.

'gargantuan' entered English via French influence (compare French 'gargantuesque'); the figurative sense 'of enormous size' was adopted into English by the 17th century and stabilized as 'gargantuan'.

Initially it referred specifically to the giant 'Gargantua' (a proper name), but over time it evolved into the general adjective meaning 'extremely large' or 'huge'.