Through large spectroscopic surveys providing chemistry for > 10^6 stars and Gaia mapping >10^9 stars, we have had an explosion in understanding the merging and assembly history of our Galaxy; this is dominated by Gaia-Enceladus/Sausage (GES) that merged 10 Gyr ago and now contributes largely to the Milky Way's inner stellar halo. I will present new work in understanding this massive contributor to the stellar halo, before it merged with our Galaxy. Specifically, I aim to understand if we can unwind the dynamical evolution of these stars and go from their present day metallicity gradient in the stellar halo to their pre-merger metallicity gradient in GES if this was already in place before the merger. To aid in this investigation, I use the AURIGA hydrodynamical cosmological simulations that contain 10 Milky Way-like galaxies with a GES progenitor and show how the change from pre-merger to post-merger metallicity gradient is affected by the time and duration of the merger, host properties, GES progenitor properties, and orbital history. Ultimately, I aim to apply this to the real data, with ongoing next generation spectroscopic surveys that will map the halo of the Milky Way most extensively such as DESI and WEAVE. This is even more exciting as JWST will be able to resolve Milky Way-like and GES-like progenitors and their metallicities at z~2, therefore putting into context how common place or unique the Milky Way's assembly and satellite population are across cosmic time.