  We discuss some general and relevant features of longitudinal gravitational
modes in Horndeski gravity and their interaction with matter media. Adopting a
gauge-invariant formulation we clarify how massive scalar and vector fields can
induce additional transverse and longitudinal excitations, resulting in
breathing, vector and longitudinal polarizations. We review, then, the
interaction of standard gravitational waves with a molecular medium, outlining
the emergence of effective massive gravitons, induced by the net quadrupole
moment due to the molecule deformation. Finally, we investigate the interaction
of the massive mode in Horndeski gravity with a noncollisional medium, showing
that Landau damping phenomenon can occur in the gravitational sector as well.
That allows us to introduce the concept of "gravitational plasma'', where
inertial forces associated to the background field play the role of cold ions
in electromagnetic plasma.
