Gigantos is the final boss of Falsion, a Famicom Disk System game.
Gigantos is a fortified battleship used as the mothership by an alien race which is invading various planets, having attacked the human colony on Pluto and taken it over without much resistance. These aliens had mastered a technology called Hyperzone Drive, something humanity was still experimenting on and planned on using to expand their territory in the Milky Way.
Without much options, the United Space Force, responsible for the space program, decides to use the Hyperzone Drive to send ships to directly attack the forces coming from Gigantos. Only one ship successfully survives the trip, the titular Falsion, which then faces Gigantos forces and confronts the mothership, destroying it for good.
Though Gigantos was destroyed, the ending implies that there may be a second Gigantos somewhere in space destroying planets.