Gaia is the Titanide personification of earth. She is the matron deity that is at the foundation of most living things, making her “mother earth”. She first originated from Chaos, together with her siblings. Unlike those siblings, she has a strong personality that features in the several stories of Greek cosmology.
Her children include Uranus and Pontus, which she has conceived with her sibling Aether, who is the personification of the atmosphere. By mixing the Earth and the Atmosphere together, Uranus and Pontus were born: the heavens and the seas respectively.
Gaia loves all of her children, especially her two firstborn. She loves them so much that she mated with them, which resulted in some interesting offspring. Through Uranus, she gave birth to the Titans and Titanides, as well as the Hekatónkheires, Cyclopes, and Gigantes.
Through Pontus, she gave birth to Neureus and Thaumos (sea gods), Phorcys and Ceto (sea monsters), and Eurybia (a goddess that personified the mastery over the sea).
Through Tartarus, she gave birth to Typhon in an attempt to get rid of Zeus. This would be the father of all monsters, and he was so horrifying powerful that even Zeus was afraid of him.
On a more comic note, Gaia was impregnated accidentally by Hephaestus as well when he tried to force himself on Athena. Oops!