  We consider ensembles of bipartite states resulting from a random passive
Gaussian unitary applied to a fiducial pure Gaussian state. We show that the
symplectic spectra of the reduced density operators concentrate around that of
a thermal state with the same energy. This implies, in particular,
concentration of the entanglement entropy as well as other entropy measures.
Our work extends earlier results on the typicality of entanglement beyond the
two ensembles and the reduced purity measure considered in [A. Serafini, O. C.
O. Dahlsten, D. Gross, and M. B. Plenio, J. Phys. A: Math. Theor., 40(31):9551
(2007)].
