Your question is the wrong way round. The concept of gravity being the force that makes apples fall is ancient. Before Newton, we knew that gravity makes the apple fall, but we didn't know what held the earth, moon and sun in a stable system.

What Newton did was to extend idea of gravity to the heavens. He did this by realising, and then mathematically proving, that the thing making apples fall is the same thing that holds the sun, moon and earth in a system.

This is what it was called 'universal gravitation'.