In a podcast mod osborne talks about death... I think it talks about it in the bird and the worm podcast. He mentions that the gods dont necessarily lose their soul. But the death of a god is so cataclysmic that it obliterates the soul in the process of killing the god. And in the last quest involving the dragonkin we learn while fighting the shadow of jas that the gods do leave an imprint on the stone. No, they forfeit their souls when they ascend. When gods die, they can either explode (Tumeken), bleed out into the world's energy (Guthix), or have their power absorbed by another, mortal, or channeled elsewhere (Bandos, Jododu Otoku, Skargaroth, V)