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Then, is the force of gravitation always the force of attraction or repulsion?
Gravity is not always attractive: Gravity is thought of as a weak boring force. But if you get fast spinning black holes and strong gravitational waves interacting, then you can get 'repulsion and attraction' effects happening out of purely gravitational forces.
Keeping this in consideration, where is the force of attraction greatest?
The strongest force of attraction is the Strong nuclear force which is present in the nucleus of an atom. This is because the force has the tendency to bind the positively charged protons together in spite of the strong repulsive force between like charges in such a small range viz. 10^-15 (size of nucleus).
According to Newton's law of gravitation, any two bodies, in this universe, attract each other with the force of gravity. So, the gravitational force between two masses is always attractive.