Ok, so with electricity we have a charged particle creating an electric field, wherein a test charge further from a field-generating particle has a higher potential energy than a test charge closer to it, and so far this is analogous to gravity, in which case we have an object creating a gravitational field, and a test mass further from it has a higher potential energy than a test mass closer to it.

 

In electricity, the test charge's ratio of potential energy to charge is known as electric potential. In gravity, I assume that analogous to this would be the ratio of gravitational potential energy to mass for a test mass, which would be the product of the gravitational constant and the distance from the centre of the source of the gravitational field. Would I be correct in assuming this?

 

Furthermore, this suggests that the gravitational equivalent of voltage would be the difference in the gravitational potential over a distance, and hence would be proportional to whatever difference there would be in gravitational constant and whatever difference there would be in distance from the centre... but since gravitational constant itself is a function of distance from the centre, would it be redundant to include?

Right, the gravitational potential is the potential energy per unit mass, so it's GM/r for anything that can be treated as a point source; near the surface of the earth changes in the potential are given by gh