  Gravitational field can cause a rotation of polarisation plane of light. This
phenomenon is known as the gravitational Faraday effect. We study the
gravitational Faraday effect of linearly polarised light propagating in the
gravitational field of a weak plane gravitational wave (GW) with $+$, $\times$,
and elliptical polarisation modes. The corresponding gravitational Faraday
rotation is proportional to the wave amplitude and to the squared distance
traveled by light and it is inversely proportional to the GW's squared
wavelength. The rotation is also maximal if light propagates in the direction
perpendicular to the GW propagation, along directions of its polarisation.
There is no gravitational Faraday rotation when light and the GW propagate in
the same or opposite directions, or it propagates along directions
perpendicular to directions of the GW polarisation. Helicity of elliptically
polarised GW gives higher-order contribution to the gravitational Faraday
rotation.
