  We present the concept of a magnetless Reflective Gyrotropic Spatial Isolator
(RGSI) metasurface. This is a birefringent metasurface that reflects vertically
polarized incident waves into a horizontally polarized waves, and absorbs
horizontally polarized incident waves, hence providing isolation between the
two orthogonal polarization. We first synthesize the metasurface using surface
susceptibility-based Generalized Sheet Transition Conditions~(GSTCs). We then
propose a mirror-backed metaparticle implementation of this metasurface, where
transistor-loaded resonators provide the desired magnetless nonreciprocal
response. Finally, we demonstrate the metasurface by full-wave simulation
results. The proposed RGSI metasurface may be used in various electromagnetic
applications, and may also serve as a step towards more sophisticated
magnetless nonreciprocal metasurface systems.
