Gan Eden is spiritual, and is not bound by physical limitations and therefore it is no place and every place. For example, where is the theory of relativity? That's a silly question, because an intellectual concept is not defined by the parameters of place or time. Only the physical is evaluated based on these factors. Gan Eden is a spiritual level which the soul experiences. In fact it is possible for a great Tzaddik (saint) to experience this level while alive on this world.