if my understanding of Unity is correct.. gravity is basically just a vector 3 variable (0.0, -X, 0.0) thats constantly applying AddForce. X is equal to your scenes gravity.
To me, I try to go with what works. Then I can go back and refine it if i deem it necessary.
It seems like your attempting to create a blackhole like object, that whenever a player or something gets near it, it pulls them towards the black hole. I've never really attempted to create a blackhole like object. But again, since we just talked about boxes, we can use them set as triggers to accomplish the goal. This is not the most efficient way, but simply the first thing that came to mind.
If we look at the attachment, lets say that square 5 is your black hole. You can create the collision triggers around it.
Refresher on triggers:
Programming: Set an FSM and have a global state for trigger enter, to apply ADDForce using a vector 3 variable to whatever it collided with.
box 1 set vector 3 to: 5, -5, 0
box 2 set vector 3 to: 0, -5, 0
box 3 set vector 3 to: -5, -5, 0
box 4 set vector 3 to: 5, 0, 0
box 5 set vector 3 to: 0,0, 0 (Nothing to apply, as they are where you want them)
box 6 set vector 3 to: -5, 0, 0
box 7 set vector 3 to: 5, 5, 0
box 8 set vector 3 to: 0, 5, 0
box 9 set vector 3 to: -5, 5, 0
By doing it this way, you are funneling all objects towards square #5
You could even get rid of the even squares, like 2,4,6,8. and just make 1,3,7,9 twice as large so the cover the same space that the even numbers one did.