If V is the gravitational potential due to sphere of uniform density on it's surface, then it's value at the center of sphere will be

Potential on surface$=\frac{-\mathrm{GM}}{\mathrm{R}}=\mathrm{V}$

At a point inside the solid sphere, ${\mathrm{V}}_{\text{inside }}=\frac{-\mathrm{GM}}{2{\mathrm{R}}^3}\left(3{\mathrm{R}}^2-{\mathrm{x}}^2\right)$

At the centre: x=0 
Potential at center $=-\frac{3}{2}\frac{\mathrm{GM}}{\mathrm{R}}=\frac{3}{2}\mathrm{V}$