Gaea is believed to be the plane's protector and its creator, and many elves and druids worship her. While never physically appearing, her existence seems to be confirmed by many individuals, and it looks like she assisted directly in protecting Dominaria during the Phyrexian Invasion.
In "The Myths of Magic"'s short story The Lady of The Mountain, Fiers helped her create the world (both Fiers and this story's canonicity are debatable). In prerevisionist continuity, she was just a green based planeswalker revered as a goddess, in a similar fashion to Freyalise.
Very little is known about her, which makes classifying her very difficult: in the early days, the name Gaea was often just added to cards that the designers wanted flavored around nature, without much thought given to the implications.
Later, Gaea was re-thought of as Dominaria's "real" goddess, and references to druids and elves being able to hear her will started to appear, as well as implications that she created the elves of the plane.
In the novels, Multani prayed to her, and though she never answered directly, he said he always sensed her answer. Molimo also worshipped her, and the Citanul druids who followed Titania claimed to follow her will: this could be interpreted to mean that all Maro-Sorcerers of Dominaria were in contact with Gaea (being presumably created by her), and served as intermediary on the plane between her and her worshippers. It is also said that during the Phyrexian invasion she transmuted fallen Phyrexians into elementals.
Other characters, like Urza and Freyalise, seem to confirm her existence in the way they speak of her. Yawgmoth believed she was in fact Rebbec that had gone through a similar transformation as he himself had experienced. Not many storyline fans believe Yawgmoth's claims, but it has muddled up the debate about Gaea quite a bit. In the novel Scourge, Karona travelled to Phyrexia and Yawgmoth told her she was the "only true goddess in Dominaria" : the meeting has been erased from continuity, as Phyrexia had already been destroyed at that point, and Yawgmoth was already dead.
Quoted or referred to:
- Flavor text for Gaea's Protector