Gaia is the name given to a supercomputer that features in the Appleseed universe.
Gaia was a supercomputer created by the government of Olympus to govern their new civilization after the devastation that affected the world. It was created to control all computer systems within the city and included remote access of a number of weapon systems. As such, it was vital to the management of Olympus's society and thus had its own automated security system. Access was only achieved through an elevator at Tartarus that allowed an approach to the Gaia supercomputer with one of the adjacent rooms containing a number of databanks. Computer access was also achieved through a number of remote kiosks scattered around the city. During its construction, it was decided that a failsafe was needed in order to sever the link Gaia had with the rest of the city and thus the ability to do so was encoded within the DNA of the Bioroid known as Hitomi. This was a highly protected secret with knowledge of it being present only within classified computer files as well as several high ranking personnel at Olympus.
Though seemingly an ideal system, there were some who did not approve of the controlled management made by Olympus. One among these groups was a terrorist faction known as the Human Liberation Front with one of their chief operatives being A. J. Sebastian. He learnt at the time that the government had completed creation of a powerful mecha weapon system called the Multi-pede Tank that required only a single operator but that future models were all going to be controlled by Gaia. The link to Gaia meant that the tank was incapable of being commandeered and thus Sebastian sought the records of the failsafe for the supercomputer. Upon learning of Hitomi, she was kidnapped and taken to one of the remote kiosks. Despite Olympus security destroying the many kiosks, a single was left behind to serve as part of a trap but Hitomi was sent into the device. This severed the link the supercomputer had to the city and allowed Sebastian to take control of the tank. With it, he intended to destroy the city but members of the police force entered into the Gaia computer where they evaded the automated security systems and found the schematics for the failsafe control thus destroying it.
With the failsafe removed, Gaia went back online and took control of the Multi-pede Tank causing it to go inactive thus ending the threat of A. J. Sebastian who was killed by the police force.