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.
Gargantus' extra-terrestrial creators look pretty similar to The Guna (right down to the green skin, three fingers, no nose, and major eyebrows), although they don't have ears. They're almost dead ringers for the Guna.