Gaia is a theoretical entity, being the Earth's sentience that can impose it's will. Gaia is able to bestow powers abilities onto people, however, she is most well known for preventing those within the Abyss from interfering with the real world through "bad luck"
The greater the disruption, the heavier the price paid is.
She is also responsible for the creation of Illusion Barriers, where supernatural powers and entities can exist without Gaia's punishment
Ren is an example of one.