The Urn of Gaia is located on the Colossus level, just before you see the Colossus' eye through the window. Its at the first block moving puzzle where there are a series of rooms and metal doors you must open. First, destroy the statue on top of the moving block. Then, when you get into the next room where there are doors on all sides of you, go into the room on your right while pushing the block. Push it all the way down the hall and then jump on top of it to access 2 blood chests and the Urn of Gaia.