What is the reaction force for the force of gravity acting on an object?
Please see the explanation.
If the object is in equilibrium, then the object is resting on something. Whatever the object is resting on is exerting a reaction force that is equal in magnitude but opposite in direction to the force of gravity.
If the object in not in equilibrium, then the reaction is the acceleration of the object in direction of the force of gravity. The magnitude is equal to the force of gravity divided by the mass of the object.