Gigan's first appearance was in the 1972 film Godzilla vs. Gigan. In the film, Gigan is summoned to Earth by the antagonist Nebula M Space Hunter aliens where he was paired with the three-headed space dragon King Ghidorah to destroy Tokyo. They were challenged by Godzilla and Anguirus, and after a long fight the two space monsters were driven away.
In the 1973 film, Godzilla vs. Megalon, Gigan was sent by the antagonist Nebulans to assist the people of Seatopia in their assault on humanity by aiding their god, Megalon, in a battle against Godzilla and the robot Jet Jaguar. After receiving a broken arm at the hands of Jet Jaguar, Gigan fled to space, leaving his erstwhile ally Megalon to face Godzilla and Jet Jaguar alone. This is the second time Gigan has abandoned an ally. This seems to indicate that Gigan has a traitorous or cowardly personality.
Also in 1973, Gigan made an appearance on Toho's television series Zone Fighter. After his battle with Godzilla and Jet Jaguar, Gigan was captured in space by the Garoga army and sent back to earth to prevent Godzilla from rescuing Zone Fighter, but he is soundly defeated by him. Left for dead, Godzilla leaves believing he is victorious, but Gigan revives shortly afterwards and battles Zone Fighter himself. In the final battle, Gigan is seemingly killed by Zone Fighter. It was made official by Toho that this indeed happened in the Showa Godzilla universe.
Gigan's latest appearance was in the 2004 Millennium film, Godzilla: Final Wars, where he is one of the main antagonists for Godzilla to fight. Long ago, Gigan once fought Mothra for the fate of the Earth and ultimately was defeated by Mothra. Years later his sarcophagus is discovered and analyzed by the human, but is suddenly revealed to be a weapon used by the Xiliens too. Gigan awakens and causes havoc and destruction in Japan, but is then sent to destroy the Gotengo in hopes of not awakening Godzilla. Although Gigan does manage to bring down the Gotengo (temporarily,) Gigan then faces off against Godzilla and is seemingly killed when Godzilla blows his head off with his Atomic Ray. During the climactic battle between Godzilla and Monster X, Mothra arrives to assist Godzilla but is intercepted by a revived Gigan, whose arms have been retrofitted with enormous chainsaws in place of his iconic blades. Mothra and Gigan resume their ancient rivalry and after a long battle that briefly interrupts Godzilla and Monster X's battle, Gigan is killed by a kamikaze attack from the flaming, grievously wounded Mothra.
Hooked arms: Gigan possesses razor-sharp hooked blades in place of hands which the monster uses to both batter and stab his opponents. In his appearance on the television series Zone Fighter, the tips of the hooks can release an explosive charge on contact with an enemy. In Godzilla: Final Wars, Gigan's arms were replaced with chainsaws after being defeated by Godzilla.
Antigravity flight: Gigan has a trio of retractable wings which is capable of allowing Gigan to fly at speeds of up to Mach 3. This normally assists in interstellar travel and is utilized very little, if at all, during battle situations. When it is used in battle, however, Gigan will often use it to engage in aerial slams to continuously topple his opponents without giving them a chance to counter, or even get up. In Godzilla: Final Wars, the antigravity flight was replaced by jetpacks, presumably to give Gigan more of a robotic feel.
Buzzsaw: Gigan has a buzzsaw built into his abdominal area. It is of considerable durability and effectiveness as it was sharp enough to cause severe epidermal damage to Godzilla and Anguirus (being the first weapon shown to breach Godzilla's skin and actually draw blood) and brought down a weakened Gotengo.
Grappling cables: In Godzilla: Final Wars, Gigan was able to fire two hooked grappling cables from beneath both of his hand blades which he used to snag onto his opponents and reel them into range of his deadly buzzsaw.
Razor discs: In Godzilla: Final Wars, the upgraded Gigan was able to fire two razor sharp disks from just above his buzzsaw. Mothra however dodged them and simultaneously released her reflective powder, which caused them to malfunction and boomerang back towards Gigan who was promptly decapitated the second time in that movie.
Eye beam: In Godzilla: Final Wars, Gigan could fire a red beam from his eye that scattered into a shot-gun like blast of energy. This ability, however was never used in the Showa films, possibly due to budgets cuts. In Godzilla vs. Gigan an unexplained blue light sparkles on his forehead only for a split second after a rock hits his forehead.
Godzilla vs. Gigan
Godzilla vs. Megalon
Godzilla: Final Wars