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.