Gedo Mazo is made from wood if i'm not wrong and it also rests on that huge flower that looks like the flower at the center from the forest that hashi could create and hashi also had a few bijuus in his possesion gedo mazo would a safe place to store them all dont u think and for nagato to be able to summin it well its the same like madara summining the kyubi if some how nagato came across it some how coz i remember wen he first summined it konan said to nago he mustnt use it meaning she saw it before somewhere so thats my view on this