I could say he was murdered by ants and Hussie would just make him die by aliens. But for a serious guess, I think that all the adults are in on SBURB somehow, since Granddad and Rose’s Mom share the same tech as Future SBURB and John’s Dad did some investigations on it.

So I think that Granddad did create the SBURB game as a way of saving the world. Jade had to get her future seeing powers somewhere, right? So he saw what would happen in the future, and in a way to prepare he traveled, got supplies, built his fortress of solitude, trained/protected Jade, etc.Sarah Conner Style.

But he is still a very weird old man. Because he had to have died when Jade knew him, so at least like 5 years ago from this point. Then he had to have it in his will to be taxidermy, so he could be revived in the game world? Which meant that Jade was the one to stuff Grandpa’s body since she was all by herself on this island.

So to sum it, gramps was a very strange doomsday-prepper.