inside a sphere?
-Gravity inside a hollow sphere is zero.
-Gravity at a certain radius inside a solid sphere depends on how much mass is contained within that radius.

In order to set Gravity in spheres in such, it's best to think in spherical coordiantes (rho, theta, phi). In this case, gravity will be constant in (theta, phi) and have the classic 1/(rho) squared behaiviour. Now you can convert this to Cartesian Coordinates to align them with the levels absolute coordinate