Gaia is the Spirit representation of the Earth, the mother of all living things. More directly, she is credited with creating the Garou and most of the Fera to be her warriors, servants and agents in both the material and spirit worlds.
Gaia is a Celestine, and one of the most powerful spirits known to exist - possibly the most powerful. 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 represention of the planet Earth, Garou and others often think of her as the entire cosmos, with the planet 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.