The scattering characteristics of random rough surfaces illuminated with a 3D converging-diverging Gaussian beam are investigated by applying the conventional Harvey-Shack theory in conjunction with 2D plane-wave decomposition. The Gaussian beam is assumed to have an arbitrary angle of incidence and to be linearly s-polarized. Using data obtained from laser BRDF measurements on isotropic random rough surfaces with low surface roughness, we demonstrate that the Gaussian beam Harvey-Shack theory is in better accordance with the experimental data than the conventional Harvey-Shack theory. The two models become identical for a large beam waist radii but are significantly different for smaller ones.(c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement