The Gaian Path is Titan's Trials area D1. The only initially available Mission here is Mission 45 (E1), which leads to the Titan's Throne. Once you have completed Missions 45–50, you can access Mission 51 from here (or any other "D" area). There is an Ochu guarding both the path to the Mission 51 Cie'th stone and a Warding Talisman. You'll find the Tonberries on the path to the Mission 45 Cie'th stone.