In Walk Two Moons, what does Gramps mean when he says, “I’m going to die in that bed, and then that bed will know everything there is to know about me”?

Old Gramps Hindle is referring to the marriage bed he's shared with Gram for so many years. He often tells Sal the story of how he came by it, always ending with the line quoted in the question. The bed isn't just an object for him; it has deep personal significance. So much of his family's history is bound up in it. The marriage bed originally belonged to his parents; he and his brothers were all born there. Each and every stage of Gramps's life is in some way reflected in the old marriage bed; so too will be his death. Gramps was born in the bed, and it is there that he will one day die. Then the bed will "know" everything about him, for it will have recorded all the most important events of his life.