A spherical object made of stone is dropped into a container of water and sinks to the bottoms of the container causing 30N of water to spill out. What is the estimated gravitational force of the object?

I do not know.

All you can say from this is that the volume of the object is the same as that of the water displaced.
30 N = 1000kg/m^3 * 9.81 m/s^2 * v
v = 30/9810 cubic meters

However we do not know how much force it exerted on the bottom of the container so we do not know what its density is.