Gaia is a monstrous, evil entity born of human greed and the power of the Spirit Stone. It was responsible for the destruction of the ancient Angelou civilization that prospered a millenium before the events of Grandia.
Gaia takes on many forms during Grandia, all of which have a gigantic, insectoid appearance. Furthermore, Gaia has tentacle-like appendages that resemble the roots of a plant, which allow it to spread its corruption across the world, destroying anything in their path.
In its first form, Gaia is nothing more than a dormant life form within a shell, awaiting the day it would awaken. It is a gigantic, brown shell with a spiral casing, with a head adorned with horns portruding out of the top. The spiral winds down to the bottom, revealing spikes.
In its second form, Gaia resembles a gigantic, grotesque, horned catterpillar. Its mouth, perhaps its most notable trait, resembles the mouths found on the Gaia Battlers. Its eyes are large, intimidating, and crimson red.
In its third, and final, form, Gaia hatches from its pupa and grows into a truly monstrous beast, resembling both a plant and an insect. Its wings have fully grown, with the exception of its bottom-left wing, which was presumably destroyed due to Leen's sacrifice. Curiously, Gaia no longer has the two eyes its previous form had. Instead, it has only one eye. It is implied that the Spirit Stone lies behind this singular eye.
Furthermore, Gaia's third form is the only one that has its insides unveiled. Its organs are unsightly and putrid, resembling a rotting corpse more than a living being. Deeper within Gaia are strange chambers which resemble Angelou architecture, which ties to its roots in the ancient civilization.
During the time of Angelou, humans became envious of the power granted to the Icarians through the Spirit Stone and tried to wield the stone's power themselves. The result is that the power of the spirits were mutated and became manifested in a perverted form: Gaia.
Gaia quickly laid waste to the once-great civilization of Angelou, and it was only because of the sacrifice of two Icarian twin sisters that Gaia was sealed away for a millenium, before the Garlyle Forces would attempt to unleash its horrifying power once again.
Gaia's presence is first felt when Justin and Feena arrive in East Elencia, after they meet and befriend the unruly teenager known as Rapp. Rapp gives them a tour of the old, abandoned village of Cafu, which has since been utterly petrified, some of its old inhabitants included. Justin calls upon Rapp to help him investigate the potential cause found in the Tower of Doom, and after befriending yet another unruly teenager, Milda, they discover the source of the cause: a strange growth the Garlyle Forces call Gaia.
Justin attempts to break the console controlling the growth of Gaia, however, the plan backfires as Gaia reacts to the presence of the final piece of the Spirit Stone and it begins to experience a rapid growth. Gaia petrifies all those in the room instantly, with the exception of Justin and his friends, due to the protective barrier of the Spirit Stone. A Gaia Battler emerges from the recently-expanded growth, which Justin and his friends engage in battle. The Gaia Battler is reduced to a seed-like object, which the party procures and takes with them back to Cafu.
Later that night, however, the Garlyle Forces besiege Cafu in an attempt to steal the Gaia Seed back from the party. Leen reveals herself to be an Icarian and, in a flash of light, disappears with the Gaia Seed.
The party's next encounter is even stranger, with the Abandoned Village of Laine. Gaia's influence effected Laine much in the same way its influence effected Cafu, though to a much greater and more twisted degree. Instead of the old village simply being petrified, Gaia's influence utterly warped the entire area. The village, no longer resembling anything of the earth, resembles a twisted alternate dimension more than anything. Strewn about the landscape are dimensional warps, otherworldly monsters, and the corrupting tentacles of Gaia. Deeper within the village are some of its old inhabitants, trapped in floating crystals. In the deepest part of the village, which resembles the innards of a fully evolved Gaia, the party engages its second Gaia Battler. It is dispatched in the same fashion as the first, and the party moves onward.
Later on, during the events on the Granduer, Justin engaged Baal in order to save Feena. In accordance with this, Justin hands over the final piece of the Spirit Stone, the key to the revival of Gaia. Baal attempted to dispose of Justin, but failed, and engaged Justin in combat instead. Baal is defeated, but he reveals the strange growth in place of his right arm underneath his cloak. Justin attempts to defeat Baal once again, but fails, and is launched off the burning ship. Feena soon follows, leaving Baal to relish the moment of acquiring the final piece of the Spirit Stone.
After a coup d'état executed by Mullen against Baal, Baal retreated into the depths of an ancient subway system built by the Angelounian people. In there is the complete Spirit Stone, Baal, and Gaia, just moments away from hatching. Justin and his friends pursued Baal, and arrived just in time to fight Baal, who is revealed to be at one with Gaia, once again, but they were too late; Gaia awakens from his thousand-year slumber and begins to terrorize the world once again. The ruins crumble due to Gaia's rebirth, nearly killing the party in the process. Due to Leen's intervention, however, the party survived.
Gaia wastes no time in spreading its roots across the land; the first of which to feel its terrifying power is the city of Zil Padon. The party pre-emptively arrives in Zil Padon in order to evacuate the city, but they are once again too late: Gaia's roots attack and lay waste to the town. The trees themselves begin to petrify, with a sandstorm accompanying it. They engaged another Gaia Battler, but the fight is cut short due to Leen eradicating it before they party got the chance to fight it. Leen is knocked out by a tentacle, however, so the party is forced to fight another Gaia Battler when it appears. Their fight is fruitless, however, for a dozen more Gaia Battlers appear once the second one is dispatched. Justin and Feena somehow combined their powers and eradicated all of them at once and halted the advance of Gaia in a burst of light, however, leading Leen to believe that Mullen's way of destroying Gaia was not the only one. The town was in shambles, but the survivors vowed to rebuild the city under the guidance of the Mogay.
Later on, Mullen attempts to halt the advance of the central Gaia growth using the weaponry of J-Base, specifically the Steam Cannon. Gaia attacks the base, but after a touching speech from Leen about the powers Justin and Feena hold, she sacrifices herself, damaging Gaia in the process. Gaia retreats back to the Icarian City, where it begins its preparations for its final evolution.
Even later on, after Justin acquires the Spirit Sword, Gaia begins its final evolution. After its growth finished, Gaia spread its seeds all across the world, corrupting all monsters that they touch. The party enters the Icarian City, and then later Gaia itself, after a fight with yet another Gaia Battler and a duel between Justin and Mullen. The Garlyle Forces, along with Sue, Gadwin, Milda and Guido aid Mullen in fighting against Gaia on the inside. The party, however, enters Gaia itself to destroy its core.
Inside Gaia, the party travels through its innards, fighting monsters along the way, in search of the core of Gaia. After fighting a heavily mutated Baal again, the party finally reaches the strange core of Gaia. A last line of defense, appears, however, with the Gaia Core, which is derived from Baal's worm-like form. Through Baal's voice, Gaia utters "We know you... we know you want something. We know you hate something. We know that all humans were born to hate what is richer than ourselves and strive to TAKE it. Even if you were to take all the riches that are left in the world your greed will NEVER be satisfied." In reference to its beginnings, Gaia-Baal attempts to break the party's morale by saying that humans are merely greedy creatures meant to hate and steal from others. Justin retaliates, saying that humans were never born to do such things, that they were meant to open a new path together. Gaia-Baal and the party fight, and the party emerges victorious, ending the General's selfish ambition, and leaving the chamber of the Spirit Stone unguarded.
In the chamber, Justin vows that humans will never rely on the power and light of the Spirit Stone, instead going through life with their own light. Justin brandishes the Spirit Sword, asking the Spirits to walk together with humans in left, and destroys the Spirit Stone. Gaia is then purified in a dazzling spectacle of lights. After it all, Gaia loses all of its insect-like characteristics, now only looking like a large, bountiful tree. Mullen comments that, not only has humanity and the world been freed from a "web of evil," but Gaia has been freed as well.