Help How to simulate Planet gravity

[josemendez]

But I still don't know how to get solver center position

solver.transform.position. That is the solver's center position.

Do solver position and set gravity position overlap?

I don't know what you mean by "gravity position", gravity is not a position but a vector. 

And what is solver in"var pos = solver.transform.position;"

"solver" is a variable of type ObiSolver, that is, a reference to a solver.

These are very basic programming/physics concepts. If you're new to Unity or game physics you will find Obi to be very hard and confusing to use, since it's aimed at advanced users. I'd recommend reading up on C# programming, transform hierarchies and basic physics (forces, accelerations, impulses, mass, etc).