Can we consider a cube as a Gaussian surface, for a point charge located at its center.since,Gaussian surface is a closed surface which has a constant electric field but in this case the both the magnitude and direction of electric field is changing at different points on the surface of cuboid. Can we use Gauss law for this surface in this condition.
Yes, any closed surface is a Gaussian surface in the sense that integrating the electric flux over it gives the charge inside the surface. The surface does not have to have a constant field strength or direction over it. However, if possible we normally choose a surface where the field strength and direction, if not constant, at least obeys a simple equation because that makes it easier to do the integral.