what is the maximum value gravitational potential and where??

 There is no maximum, as the zero point of potential energy is arbitrary. If one chooses the usual zero point, however, so that the formula

U = -G(m1)(m2) / r

can be used to describe the potential energy in a system of point charges (or radially symmetric spheres) as a function of their separation distance r, then the maximum value is zero, at infinite separation.

Of course, if your system isn't like that, or you choose a different zero point, your answer will be different