How can I set my scene's gravity to a game object?

Basically I want to take a game object (for example a giant sphere) and set is as the scene’s center of gravity (or at least emulate this effect through code) so that I can drop an object in my scene- such as a cube- and have it fall to the surface of the giant sphere. And perhaps maybe have the cube move around on the surface of the sphere.

You can’t do that with Unity settings, you have to implement your own mechanics to simulate this gravity. There is a similar question here.