The game takes place on a past earth, though obviosly, most of the events that take place are fictional. There are however many real world areas such as the Great Wall of China and the Egyptian pyramids.
While Illusion of Gaia has a large cast of characters, Will, Freedan, and Shadow are the only playable characters in the game. They each have unique abilities, and certain areas are impassable without a specific character. The characters gain techniques as part of the story. Will's techniques are all based on reaching new areas with incidental combat applications, while Freedan's techniques are more combat-oriented. Shadow arrives late in the game, but being such a powerful character, he causes Freedan to be nearly obsolete.
Combat is relatively simple. Characters share the same health and defense scores, but have different levels of strength. Freedan, for example, does noticeably more damage, and has a longer reach than Will. In turn, Shadow does more damage than Freedan. Attacks are almost exclusively melee, using Will's flute, Freedan's sword, or Shadow's pseudopod. Enemies' health bars appear upon attacking, displaying as a series of red spheres that represent hit points. Bosses cannot be revisited, and enemies do not reappear unless Will loses all his lives or completely exits an area and then returns.
Illusion of Gaia has a general design that is uniquely simple as RPGs go. The game eschews the experience system of typical RPGs; instead, the game has a more novel system for advancing the player character's statistics. Defeating all enemies in a room earns the player a permanent stat bonus in the form of a jewel. These jewels boost attack, defense, or health power. While returning to a cleared area will cause enemies to reappear, the bonuses for defeating them again do not. Also, after an enemy is killed, it will leave a stone—either a small or a great one. Collecting 100 of these allows you to restart closer to where you died with all enemies still defeated.
Also, the game has no currency or equipment systems. There is only one healing item (herbs), and only a small number of those in the game. Unlike most games of its type, formerly visited areas become impossible to revisit almost immediately after the story progresses beyond them until the last third of the game, though areas from the first two-thirds of the game remain inaccessible. The only side quest in the game, finding Red Jewels, then becomes impossible to complete if the player fails to find them before advancing the story.
Like most RPGs, the game has only one difficulty setting. Saving is accomplished at Dark Spaces located throughout each level—including areas without enemies, such as Will's hometown. Will can recover lost health within the Dark Spaces, and occasionally switch forms or gain new abilities.
When the game begins, Will stumbles into a "Dark Space" where he meets a strange being called Gaia. Gaia tells Will that he must leave his home and save the world from a coming evil. A comet is approaching, and it will bring ill fortune to the world. As he travels, Will gains the ability to change into other forms, each with special powers: Freedan, a dark knight, and Shadow, a solid form of energy.
It is later revealed that the comet is in fact an ancient weapon used during the last Blazer War and has the power to change the shape of the world. In the ruin of Ankor Wat, it is discovered that when the comet approaches the Earth this time, the comet's light will change the world into that of the modern world.
Will and his friends travel the world and collect artifacts known as Mystic Statues. At the climax, Will and Kara reach the Tower of Babel, where it is then revealed that Will is the Dark Knight, and Kara is the Light Knight. The two knights join to form Shadow and use the ancient statues to release the ultimate power, the firebird.
Attacking the comet directly, which soon manifests itself as Dark Gaia, Will and Kara manage to destroy its power, returning the world to normal. The spirits of Will's parents inform them that the world will return to normal and that no one will conserve any memories of the adventure. Saddened by that fact, Will and Kara join one last time as Shadow to return to Earth.
The final scene is left ambiguous. All of Will's friends are depicted in what appears to be a modern-day school, implying that even if they forgot about their time together, they remained friends in the "real" world.
Illusion of Gaia is a spiritual successor to the game Sould Blazer and spiritual predecssor to the game Terranigma.
Quintet reported that Illusion of Gaia sold 200,000 copies in Japan, 300,000 copies in North America, and 150,000 copies in Europe. Although it only sold a combined 650,000 internationally, it still manages to hold a cult following to this day.