GaussGravLaw

Show that the Gauss Law for Gravitation is accurate for a point mass M at the origin.

Intuitively - the sum total gravitational flux thru any surface is proportional to the enclosed mass contained within the surface. 
This makes intuitive sense.  Suppose we have a small sphere that contains a mass M.  The total gravitational flux thru the surface of small sphere is the same as the total gravitational flux thru the surface of a large sphere. The magnitude of the flux over the surface area of the small sphere is greater (larger) than the magnitude of the flux over the surface of a large sphere.   But the sum is the same. 

Derive Newton's Law of Gravitation from Gauss's Law of Gravitation