According to Georgia State University, gravitational potential energy is the energy an object possesses because of its position in a gravitational field. This is most commonly in reference to an object near the surface of the Earth, where the gravitational acceleration is assumed to be constant at about 9.8 m/s2.
According to Georgia State University, the general expression for gravitational potential energy is equal to the work done against gravity to bring a mass to a given point in space. The potential energy near a planet is a negative quantity, since gravity does positive work as the mass approaches the surface. Once a mass is near a large body, it is trapped until something can provide enough energy to allow it to escape.