Deep in a trench within the Earth Country, near the coast of the Fire Nation, is the Gong Jen HQ. You'll find a hidden door, although unfortunately for the security of Gong Jen, it's not very easy to penetrate. For those who don't want to go kleptomaniac, most members of the group have keys designed to open the door. The door is, actually, very hard to pick, although metalbending would be a very easy way to infiltrate the base.