Gravitational potential energy:

The energy that an object has due to its height above a reference point is known as the GRAVITATIONAL POTENTIAL ENERGY.

The work done in moving a unit mass from infinity to a point in a gravitational field is called the gravitational potential at that point.

It is a negative, scalar quantity. The unit is JKg^-1.

It is easy to see how an object on which the force of gravity acts if we consider two masses m and m' linked by a massless cord passing over a frictionless pulley

If the mass, m, which is slightly greater than m' and is held at a height h above ground level were to be released, it would fall towards the ground. In the process, the mass m' would rise against the force of gravity, m'g, on it.
When m reaches the ground the work done on m' would be the product of the force required to lift it, m'g, and the distance moved, h, that is, m'gh

At the start of the experiment the weight mg has the potential to do work, or POTENTIAL ENERGY due to its location in the gravitational field relative to some reference position. In this case, a height, h, above the ground.

The GRAVITATIONAL POTENTIAL ENERGY acquired by a weight, mg, in lifting it a distance, h, above the ground is equal to the work done in lifting the weight, that is, mgh.