In a gravitational field, there is a vertical force directed downwards on all bodies called "weight" of the body. In order to raise a body to a different height, a force equal to the weight must be applied in the upwards direction. By definition, this upward force does work on the body. We say that the "gravitational potential energy"text of the body has been increased, by an amount equal to the work done i.e. The more work done in lifting the body the more the gravitational field strength.
