The marble will roll down the slope and go up on the opposite side to the same height from which it was released.
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It is difficult to achieve a zero unbalanced force in practical situations due to the presence of frictional force acting opposite to the direction of motion.
Galileo’s thought experiment suggests that no net force is needed to sustain the uniform motion of the marble; an unbalanced (external) force is only required to change its motion.
If the right-side plane were ultimately made horizontal, the marble would continue to travel forever, trying to reach the same height from which it was released, assuming no friction is present.
As the right-side plane’s slope is gradually decreased, the marble travels further distances to reach the same height from which it was released.