  We unveil the key-role of dimensionality in describing chiroptical properties
of molecules embedded inside an optical Fabry-P\'erot cavity. For a 2D-layer
configuration, we show that the interplay between molecular chirality and
spatial dispersion of the cavity-modes, results in a gyrotropic coupling $\chi$
at the origin of a differential shift in polaritonic energy-spectra. This
differential shift is proportional to $\chi$, while for 3D bulk-aggregate
configurations it is shown to vanish. We interpret physically the former
2D-chiral effect by analogy with the classical Newtonian motion of a fictive
particle in presence of 3D restoring force, and static magnetic field. The
gyrotropic coupling is shown to directly perturbate the anholonomy angle of the
classical trajectories, and the fictive particle undergoes cyclotron gyrations
upon entering the ultrastrong light-matter coupling regime.
