Edward Cobert was a scientist working for S.H.I.E.L.D. and heading a program to develop new super-humans that would control or eliminate existing super-humans on behalf of the government. Cobert subjected himself to his own experiments and became a destructive, dim-witted giant dubbed Leviathan who proceeded to go on a rampage before being subdued and imprisoned by S.H.I.E.L.D.. The Secret Empire, who had agents also held in SHIELD custody, arranged for Leviathan's release, and his subsequent rampage in the streets of New York was stopped by the Defenders, but not before the Secret Empire secreted their agents away.
Leviathan was somehow also freed by the Secret Empire, for when the Defenders managed to infiltrate the organization's headquarters, Leviathan helped capture the team. Even so, the Defenders escaped and overthrew the Empire. Later, the U.S.Agent (as Captain America) and Battlestar (as Bucky) brought down another version of the Empire, battling Leviathan, its guardian.
Cobert next became a pawn of the evil Mad Thinker, who gave him the new costumed guise of Gargantua. The Thinker hoped that Cobert would be publicly defeated by the Wasp and Wonder Man, who were engaged at the time in high-profile protests against the government's first proposed legislation regarding superhero registration/regulation. As planned, the heroes easily subdued Cobert, and thus the Mad Thinker helped sway public opinion against the legislation.
Gargantua was recruited to be a member of Doctor Octopus's version of the Masters of Evil. The group attempted to storm the mansion headquarters of the Avengers when the team was away on a mission. Unknown to the villains, this feat was made easier due to the fact that many of Earth's super-powered individuals were being replaced by monstrous doppelgangers of themselves. The Masters of Evil were thwarted by the Guardians of the Galaxy who were visiting the mansion, but both teams were forced to band together to fight their doppelgangers. After defeating them all, Doctor Octopus hoped to continue the battle against the Guardians, but Gargantua and the others refused to turn on the heroes who had helped them survive and disbanded after forcing Doctor Octopus to flee.
Gargantua later accompanied Titania (Mary MacPherran) to Hulk, Arizona, a town renamed after the well-known hero in hopes it would become a tourist attraction. The villains thought the Hulk was actually there and leveled the small town, with help from the Abomination (Emil Blonsky), searching for him. Satisfied Hulk was not there, they left. Gargantua was arrested soon after and imprisoned at the Vault but took part in a riot orchestrated by the Wizard (born Bentley Wittman) after the warden refused to give fellow inmate Terraformer the necessary plant life he needed for symbiotic survival. The uprising was quelled once Marvel Boy (Vance Astrovik) brought Terraformer a potted plant, and the warden agreed to introduce certain freedoms to inmates on an individual basis. The Vault was subsequently destroyed during another prison break.