So gravity is helpful here in two ways as far as I can see: * it causes the marbles to warp the sheet * it causes the marbles to move in a direction that a

So gravity is helpful here in two ways as far as I can see: * it causes the marbles to warp the sheet * it causes the marbles to move in a direction that approximates how an object in spacetime will take the shortest routes between two points, taking into account curvature. The limitations are: * it's 2d * the marbles lose energy to friction with the sheet * the marbles lose energy to the elasticity of the sheet So the exact orbital paths aren't accurate since they're losing energy. The warped sheet sets up a situation in which the marble's behavior of seeking the lowest energy position (downward) happens to emulate the behavior of objects in space seeking to travel the shortest distance between two points.