Gargantua means a person of great size, such as a giant.
There is also a 16th century pentology of novels by French Renaissance writer François Rabelais called Gargantua and Pentagruel. The main character, Gargantua is a Giant.
Five Gargantuas appear as optional bosses. They are artificial demons made by Dr. Thrill in his lab, which is located in the Sewer dungeons. After defeating one, a Dolly Kadmon appears from its body. All Gargantuas are Dark-Neutral aligned.
Gargantua Zero (ガルガンゼロ, garuganzero) has frozen the harbor warehouse where Dr. Thrill's lab has been relocated to, having been ordered by her master to guard the place, and must be fought to unfreeze it and gain access to Thrill's lab.