Gaia Temple is located directly under the city Nanai and can only be accessed through Nanai's castle. It houses the Quake Control along with the Earth Key.
After clearing monsters out of Camlon Castle, Ryu learns about the Quake Control in Nanai. After sneaking past the Dark Dragon Army soldiers Ryu enters Gaia Temple. Making his way through the temple Ryu finds the Quake Control being used by a knight with plans of world conquest. After defeating the knight Ryu takes the Earth Key from the Quake Control which causes the machine to blow up. The explosion destroyed both Nanai and Gaia Temple.