In discrete differential geometry, it is widely believed that the discrete Gaussian curvature of a polyhedral vertex star equals the algebraic area of its Gauss image. However, no complete proof has yet been described. We present an elementary proof in which we compare, for a particular normal vector, its winding numbers around the Gauss image and its antipode with its critical point index. This index is closely related to the normal degree of the Gauss map. We deduce how the number of inflection faces is related to the numbers of positive and negative components in the Gauss image. The resulting formula significantly limits the possible shapes of the Gauss image of a polyhedral vertex star. For example, if a triangulated vertex star is in general position and its Gauss image has no self-intersections, then it is either a convex spherical polygon if the curvature is positive, or a spherical pseudo-quadrilateral if the curvature is negative.