The Gravitational Potential Energy of an object depends on how high that object is above a surface. It depends on its position. When released the gravitational potential energy will diminish and the object will gain kinetic energy as it falls under gravity. It can be used as a store for energy (hence the word 'potential').

When you lift something up you have to do work against gravity. The work that you do is transferred to the object you lift as GPE. You expend energy to do the work and some of that energy becomes stored in the lifted object (the rest is usually 'lost' as heat).

Work done = force x distance moved by the point of application of that force

The force in this case is the weight of the object = mg (the object's mass x the gravitational field strength = the object's weight)

The distance moved is the vertical distance as this is the direction in which the force of gravity acts - the vertical distance it is lifted is the difference in height - h

So the Gravitational Potential Energy gained by the object is mgh

E_P = mgh