Gigan is an enemy to Godzilla who first appeared in Godzilla vs Gigan, where he fought alongside King Ghidorah. Then appeared in Godzilla vs Megalon, where he fought side by side with Megalon. After years off the screen Gigan reappeared in Godzilla: Final Wars. His body was found after being defeated by Mothra 1200 years ago. He was considered Mothras arch enemy, fighting the winged protector for over decades. When Gigan was restored, he came in counter with Gotengo. After the alien cyborg and the warship fought the Gotengo was sent to Antartica to free Godzilla and Gigan chases after them.
When Gotengo freed Godzilla, Godzilla and Gigan brawled it out in the frozen tundra. One atomic breath later Gigan was killed. The mighty cyborg had fallen. But it is not the last we see of Gigan.
Following his decapitation in Antarctica, Gigan was repaired and his arsenal was enhanced. The Xiliens repaired him giving him Buzzsaws in replace of the hook blades. Projectile buzzsaws. And more armor.
Overall in personality Gigan is a fierce brutal fighter and is very violent, but Gigan is also a big coward and retreats and runs away from a lot of battles in the Toho era.
Gigan's orgin has long been surrounded with mystery but what is known is that Gigan was born on an alien planet where he was a originally not a cyborg but a large animal that looks like the fusion between a bird and a dinosaur. The only thing that gives information on what Gigan used to look like was an unnoficial action figure of a pre-cyborg Gigan. Either Gigan was killed by the M Space Hunter Nebula Aliens who turned into cyborg. They used Gigan to help them conquer earth ( which they didn't), this is where you see the cyborg in Godzilla vs Gigan (1972).
Gigan has a laser that fires from his forehead. It is a device on the forehead. It was able to be destroyed when Godzilla threw a boulder at it breaking it off. Later in the Final wars it comes from Gigan's eye. And it has a buckshot like effect. When it hits the target it spreads over the targets body. And is officially known as the "Cluster light beam cannon"
Also the cyborg has 2 projectile buzzsaws that come out of his chest,which he shoots at Mothra in Final wars.
In the Showa era he he has hook blades that are able to cut threw the Gotengo and Godzillas flesh. After his repairs he was given buzzsaws that were able to cut Mothras wings.
Gigans Roar sounds like a mechanical shriek mixed with a chirp. Unlike Godzilla, Gigan has a short "roar" and in the Toho era, Gigan's chirp stops soon after gigan is done making the sound, but in Godzilla Final War, the character releases a slightly elongated chirp that has a slight echo to it.
Monster Planet Novels
Here Gigan is found in hibernation off the coast of siberia, and was brought by the United Earth forces to fight Godzilla, it was beat by Godzilla and had parts of his body removed in the fight, whenever this happened they would replace whatever part of his body was lost with mechanics and send him back to fight. Gigan actually did this quite a lot, but this isn't your ordinary Godzilla, this is Godzilla Earth which is one of the most powerful incarnations of the creature so Gigan eventually took so much that he died in batlle with the Kaiju. They took whatever was left of Gigan and sent his remains to Siberia as a monument. This shows how Gigan died as a hero in stead of the villian it is usually portrayed as. (This is a novel interpretation based on the Godzilla Earth series and not a comic book interpretation.)