Changing global gravity is easy, just set Physics.gravity as a vector with the direction and magnitude you desire.
But if you want to apply different gravity to different objects depending on where they are, you’ll need to do that on a per-object basis. Off the top of my head, my first thought is to add a Rigidbody and a ConstantForce component to all objects which can be effected by gravity. Then use a bunch of triggers in your different gravity areas to set that ConstantForce to point in the desired direction for that area.