My take is that the question's nonsensical, impossible to answer.
The game presents a procedure for you, the players, to follow. Follow it and you'll create stories of a certain type, and you'll (probably) have fun doing it.
The "physics" and "metaphysics" of the "game world" - I don't even know how to talk about such impossible beasties. There's no "game world," even in your game, just some fictional stuff and events. No "physics" or "metaphysics" underlie them, you create them out of your heads following the procedure I describe.
In the "game world" does "God" "really exist"? You might as well ask whether the clocks in "the world" of Salvador Dali's paintings still tell time. "Do their little gears and stuff work, when they're all floppy like that?" All I can do to answer is blink stupidly. The what?
Chew on that.