Who or what is Gaea?
The MTG Wiki page about Gaea summarizes what is known about her. In part, it says this:

Gaea is a goddess worshiped on Dominaria.
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.