  We analyse a set of cosmological magneto-hydrodynamic simulations of the
formation of Milky Way-mass galaxies identified to have a prominent radially
anisotropic stellar halo component similar to the so-called "Gaia Sausage"
found in the Gaia data. We examine the effects of the progenitor of the Sausage
(the Gaia-Enceladus-Sausage, GES) on the formation of major galactic components
analogous to the Galactic thick disc and inner stellar halo. We find that the
GES merger is likely to have been gas-rich and contribute 10-50$\%$ of gas to a
merger-induced centrally concentrated starburst that results in the rapid
formation of a compact, rotationally supported thick disc that occupies the
typical chemical thick disc region of chemical abundance space. We find
evidence that gas-rich mergers heated the proto-disc of the Galaxy, scattering
stars onto less-circular orbits such that their rotation velocity and
metallicity positively correlate, thus contributing an additional component
that connects the Galactic thick disc to the inner stellar halo. We demonstrate
that the level of kinematic heating of the proto-galaxy correlates with the
kinematic state of the population before the merger, the progenitor mass and
orbital eccentricity of the merger. Furthermore, we show that the mass and time
of the merger can be accurately inferred from local stars on counter-rotating
orbits.
