Gargantus are two unrelated villains from Marvel comics.
Gargantus was created by an unnamed extra-terrestrial race. Having scouted the Earth 80,000 years ago, they constructed their robot after the form of the natives they had previously encountered, Neanderthal-like cave men. In the modern era, two of the aliens traveled to Earth and sent Gargantus to Earth to lead their takeover. Gargantus arrived in the city of Granville, where it used its mesmerization power to take control of the residents. It then forced them to build a wall around the city, preventing anyone else from entering or leaving.
The invasion attempt might have succeeded, but one of the residents of Granville, Marion, had had a date with Tony Stark, and missed it due to her entrapment by Gargantua. This led Stark to investigate and learn of the wall around Granville. Investigating further, as Iron Man, he tunneled into the city and learned of Grangantua when the residents, under his control, attacked him. Iron Man fled the residents to avoid hurting them, and then used a loudspeaker to challenge Gargantus and draw him out. After a brief battle, Iron Man surmised that Gargantus was a robot and surrounded him with some of his most powerful magnets, tearing him apart with their opposing forces.
Iron Man then sent his magnets after the dark cloud hovering over the city, correctly guessing that Gargantus' creators were using it as a cover for their spaceship. Fearing destruction of theit ship from his magnets, the aliens fled, reporting their visit as a failure.
In the Early 1950s, inventor and oceanographer Dan Parmenter developed an experimental bathysphere capable of descending to previously inaccessible depths. Though Parmenter set out on his inaugural dive alone, he soon discovered his young son Billy had stowed away on the craft. It was then that the father and son became the first humans to ever see the gigantic Gargantus. Parmenter quickly surfaced, but a curious Gargantus followed them to the surface. Now on dry land Gargantus declared that the he had made the greatest of discoveries: that the land masses above the sea were not desolate as his people believed, but populated!
While Gargantus did not initially seem hostile, his sheer size and bulk lead to tremendous damage and havoc. The military opened fire on Gargantus, but he proved invincible to all conventional weaponry. Striking back, Gargantus announced his people would conquer the surface. After a brief bout, however, Gargantus was winded, and then collapsed. It was then that Parmenter realized the air was too thin for the amphibious titan, and he had, in essence, suffocated. The military released the Gargantus back into the sea, where it was presumed that he would once more be able to breathe and recover.
As time passed, the people grew less fearful of Gargantus, and wished to return to exploring the depths from which he came. However, Bill Parmenter—now fully grown and guardian of his father's technology—refused to allow anyone to use his father's bathysphere, or to share the technology. Bill was widely seen as either a coward for still being afraid of Gargantus, or a pathetic failure who could not live up to his father's legacy. Eventually, another group of oceanographers was able to duplicate Parmenter's technology (or some similar technology). Ignoring Bill's warnings, they repeated Dan Parmenter's dive in 1961 and again encountered Gargantus.
When Bill brought his father's bathysphere out of storage to follow his colleagues, he found Gargantus had captured them and was about to crush their craft in his enormous hand. When Gargantus saw Parmenter's familiar craft, he discarded the newer vessel to pursue. Parmenter dispersed a thick black chemical into the water, creating a temporary "smokescreen" while he worked feverishly with the bathysphere's mechanical claws to untangle the other craft's line. Parmenter finished without a second to spare, and the two bathyspheres ascended with Gargantus following close behind.
At the surface once more, Gargantus attacked a passing freighter and then came ashore at the pier. There he encountered (and recognized) Parmenter. Gargantus demanded to know where Bill's father, Dan Parmenter was; upon learning of his death 18 months prior, he charged Bill with the task of informing humanity that Gargantus had returned—this time to stay and to rule. Bill refused to help Gargantus in any way, and an enraged Gargantus razed his waterfront warehouse in an attempt to destroy him.
Gargantus then departed to begin his reign of terror, announcing that he would conquer the continents one at a time. After tearing a highway from the ground and leveling a few buildings on the outskirts of town, Gargantus was engaged by the military. When conventional arms failed (again) and it was realized that Gargantus had become accustomed to the surface's thin air, Bill challenged him to a one-on-one fight in his home territory. A gloating Gargantus agreed.
Bill had hoped to descend to a depths that even Gargantus could not withstand, but the increased pressure seemed to have no effect on him. When Gargantus began to crush the bathysphere in his bare hands, Bill used his weapon of last resort, and sent a tremendous current flowing across the hull of his ship. Amplified in the dense water, the current rendered Gargantus comatose, and he was left for dead.
Later, however, Gargantus was seen alive and captured in Collector's zoo beneath Canada. When the Mole Man attacked the Collector in his lair, the security in the Collector's zoo failed, and the monsters were released. The stampede of monsters eventually arrived in New York, and were engaged by the Thing and the Hulk. Joined by the Beast and Giant-Man, the four heroes defeated the horde of monsters, including Droom, Gargantus, Groot, Grottu, Grogg, Rommbu, Taboo, Tragg, and Vandoom's Monster. Afterwards, the Hulk hurled all the monsters through a portal and into the Negative Zone.
Though Gargantus has not been seen since, Several of the other monsters sent through the Negative Zone have since been seen again on Earth. It is possible that Gargantus too has returned, perhaps to his realm beneath the sea.