Can a gravitional force be repulsive in nature
It is not 100% correct to say that gravitational force is always attractive. Dark energy results in a kind of gravitational repulsion which is why the universe is currently experiencing an accelerated increase in the expansion rate - it is as if the gravitational force of the dark energy is pushing the universe apart. Now the leading candidate for dark energy is that it is a very small constant positive vacuum energy density which therefore exerts negative pressure. For how constant positive vacuum energy produces negative pressure. However there is no way for physical objects in our universe to create negative pressure, so you cannot have gravitational repulsion between two different physical objects.
The other way you could have repulsive gravitation would be to have an object that actually has a negative mass or negative energy density - it would push away ordinary positive mass objects. However this seems to be impossible in our universe - even antimatter still has positive mass. So in our universe the only example of gravitational repulsion appears to be from the negative pressure effect of dark energy. So the reason why physical objects cannot have repulsive gravitation is because negative mass/energy objects are impossible in our universe.