Grey spoke with both his father and his mother about his plan to use himself as bait to draw most of the Gnomes into the God's tomb. Not just that, with the speed at which he is improving, the Gnomes would not want him to break through to the God Plane since even then, their God Plane Elementalists that were overseeing their world would be in danger.
Grey is already very dangerous as it is. With his current strength, only Gnomes in the Mid stages can stand against him and the chances of them defeating him is also very slim, so they needed Late stage Sovereigns, powerful ones that will be able to quickly dispatch him. They all knew he had a lot of tricks up his sleeves, hence the reason he has been able to escape every assassination attempt and even the few times he fought against stronger opponents, he tends to always escape.