Gravitational force is the attractive influence of objects on each other. The strength of the force can be calculated using Newtons law of universal gravitation. The strength of the force depends on the mass of the two objects and on the distance of the two points of mass. The heavier the objects are or the closer they are together, the larger the force. The energy produced by this gravitational attraction is roughly the force (described above) times the distance of the two objects. If you are interested in more details how to precisely calculate the energy, you can look it up in any physics textbook on mechanical work/mechanical energy.

Using a hammer to pound in a nail is a simple example where gravitational energy is used. The potential energy and gravitational energy that is stored in the hammer by lifting it and then lowering it onto the nail is transferred into the nail and used to drive the nail into the material. On a more global scale, hydroelectricity is a way of storing gravitational energy and then releasing it. Water gets retained in a reservoir behind a dam (storage) and when electricity/ energy are needed, it is released into a lower lying lake or river.

Your question about energy storage (in a broader sense, not only for gravitational energy) is a very important one and a lot of scientists all around the world are trying to think of a good way to store energy without losing it over time. You are not alone in pondering this problem.