To a relativist, who can watch the motion of the marble on the bed from close up, it is obvious that there is no force at all. There is just the bending of the bed, which forces the marble to move in a curved line. To the relativist, there is no pull, there is only a push, exerted by the curved bed on the marble.
But this makes no sense. In the analogy the reason for the marble to roll across the curve toward the ball is the downward pull of gravity. Therefore, planets would have to be in absolute relation to an up or down. Probably, it's just the limitations of the analogy. If space bends in three dimensions it would answer that objection.
