Gaia is the Earth and related Realms, in both a physical and a spiritual sense; the Mother Goddess of all living things.
Gaia is a Celestine, and one of the most powerful spirits known to exist - possibly the first, and most powerful. She is credited with creating the Garou and most of the Fera to serve as her warriors and agents both in the material and spirit worlds.
Opinions differ over her exact relationship to the Triat (some say she created them, others the opposite), but she clearly has ties to Helios and Luna. While mages generally see her as a spiritual representation of the planet Earth, Garou and others often think of her as the entire cosmos, with the planet as just her most important and vital aspect.
As is the case with other spirits of God-like power, Gaia is almost never encountered directly; instead she communicates with her children and other supplicants via minor spirits under her command. She has many aspects and is known by a variety of names; these include the Emerald Mother, the guise in which she is known to the shen of the Middle Kingdom.
Gaia cannot be contacted directly, and resides within the Summer Country, blissfully dreaming of life. During one of the Apocalypse scenarios, servants of the Wyrm known as the Midnight Shadows search for her, and the player's pack can follow them to save Gaia. If they save her, the pack has the option of finding her a new hiding place, or bringing her to battle so that the Garou can defend her. In another scenario, her heart will be cut out if a tribe of Garou fall to the Wyrm and succeed in their plans.