  The Gaia-Sausage-Enceladus~(GSE) stands out as the largest known ancient
accretion event in the Milky Way~(MW) history. Despite this significance, the
parameters of its progenitor galaxy are still poorly constrained. We identify
GSE stars from the APOGEE DR17 using Gaussian mixture models and recover a
negative radial metallicity gradient for the GSE debris within the MW stellar
halo, with a magnitude of $\approx -0.014^{-0.002}_{-0.022}$ dex/kpc. We argue
that this gradient reflects the radial metallicity gradient of the GSE galaxy
progenitor before it was disrupted by the MW. By investigating the cosmological
HESTIA simulations and $N$-body models of galaxy mergers, we constrain the
radial metallicity gradient of the GSE-progenitor to be $\approx
-0.1^{-0.06}_{-0.15}$ dex/kpc. We, therefore, propose that a chemical tagging
of accreted stars using their integrals of motion, although they are not
conserved during mergers, provide essential information about the structure and
the past of systems accreted onto the MW.
