The Gaia Temple civilization is another of the many ancient societies which left their mark on Sonic's world in the form of ruined, long-forgotten temples. Relevant to the plot of Sonic Unleashed, the Gaia Temple civilization was (predictably) responsible for constructing the seven Gaia Temples at different points of the globe. Their legacy also includes the Gaia Manuscripts - records of Dark Gaia's previous emergance from the core of the planet, "some tens of thousands of years ago". Presumably, the Gaia Temple civiliation died out shortly after compiling the Manuscripts, in the ensuing battle between Light Gaia and Dark Gaia.
In the present day, Dr. Eggman gets his hands on the Gaia Manuscripts and decides to wake up Dark Gaia pre-emptively (using a giant orbital laser) to power the construction of Eggmanland. Professor Pickle of Spagonia University, the world's leading authority on the ancient civilization, is kidnapped by Eggman early on in Unleashed, along with his Manuscripts. After recovering Pickle, and the Manuscripts, from an Eggman base in Mazuri, Sonic, Chip, and Tails are sent to seek out the Temples of Gaia, which contain altars to re-charge the Chaos Emeralds. During the last boss, Chip levitates the temples out of their foundations and assembles them together to form the Gaia Colossus; whether this was their intended purpose, or just Chip being creative, is never specified.
The Gaia Temple civilization joins a long line of other extinct societies who left ruins on Sonic's world; from the Knuckles Clan and Nocturnus Tribe to the Babylonians and Black Arms. The Gaians, however, potentially have the distinction of being the oldest, from tens of thousands of years ago, as opposed to just three or four for the echidna civilizations. What species the temple-builders were is never actually addressed, although the scarcity of anthropomorphic animals in Unleashed may imply they were human.
Dark Gaia minions: