Geezer: an easier way to think about it to say that where there's a gravitational field, the properties of space have been affected by the central matter/energy of a planet. This is what Einstein said:

*"According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials g*_{μν}).."

The important point is that in a gravitational field, space is not homogeneous, and because space has "vacuum energy", this energy is not uniform. Ergo the space where a gravitational field is, has energy above and beyond that of free space.

NB: people often describe the gravitational field as negative energy, but this is an error related to "immersive scale-change" and whether two bodies falling towards each other truly gain energy.