Gierdful's Hidden Chamber of Odin is in the Foothills, the area between the Lake of Nine and the Mountain, accessed by going through the Vanaheim Tower. From Sindri's camp you'll climb up a cliff and pass through several cursed smoke totems, then come upon a tunnel to the left that leads to The Mountain. Instead of doing that, look for another tunnel to its right to find a door with an ice seal on it.