We numerically study the optical properties of metal-insulator-metal resonators and metasurfaces, emphasizing the presence of gap-surface plasmon (GSP) resonances and their connection to the optical response. In relation to birefringent metal-backed metasurfaces, we show how a combination of metal nanobrick and nanocross elements allows one to fully control the phase of reflected light for two orthogonal polarizations simultaneously. The approach is exemplified by the design of a gradient birefringent metasurface that reflects two orthogonal polarization states into +2 and -3 diffraction order, respectively, with a reflectivity up to ~ 80% and in a broad wavelength range around the design wavelength of 800 nm. Finally, we introduce the concept of metascatterers, which are wavelength-sized polarization-sensitive scatterers.