Why is the gravitational potential at the surface of a hollow sphere equal to the gravitational potential inside the sphere which is $-\frac {Gm}{r}$ ? Does this mean that the potential is the same at every place inside the sphere?
N.B > The hollow sphere generates the Gravitational field
Yes, inside the hollow shell the potential is the same anywhere, thus the net gravitational force on a test mass inside the shell is zero. This the famous shell theorem of classical gravitation theory, which is explained in many textbooks (see for a description at this link). The reason for this is basically Gauss's flux theorem for gravity.
