  Recent observations of the Milky Way (MW) found an unexpected steepening of
the star-forming gas metallicity gradient around the time of the
Gaia-Sausage-Enceladus (GSE) merger event. Here we investigate the influence of
early ($t_{\mathrm{merger}}\lesssim5$ Gyr) massive
($M_{\mathrm{gas}}^{\mathrm{merger}}/M_{\mathrm{gas}}^{\mathrm{main}}(t_{\mathrm{merger}})\gtrsim10\%$)
merger events such as the Gaia-Sausage Enceladus merger in the MW on the
evolution of the cold gas metallicity gradient. We use the NIHAO-UHD suite of
cosmological hydrodynamical simulations of MW-mass galaxies to study the
frequency of massive early mergers and their detailed impact on the morphology
and chemistry of the gaseous disks. We find a strong steepening of the
metallicity gradient at early times for all four galaxies in our sample which
is caused by a sudden increase in the cold gas disk size (up to a factor of 2)
in combination with the supply of un-enriched gas ($\sim0.75$ dex lower
compared to the main galaxy) by the merging dwarf galaxies. The mergers mostly
affect the galaxy outskirts and lead to an increase in cold gas surface density
of up to 200% outside of $\sim8$ kpc. The addition of un-enriched gas breaks
the self-similar enrichment of the inter-stellar-medium and causes a dilution
of the cold gas in the outskirts of the galaxies. The accreted stars and the
ones formed later out of the accreted gas inhabit distinct tracks offset to
lower [$\alpha$/Fe] and [Fe/H] values compared to the main galaxy's stars. We
find that such mergers can contribute significantly to the formation of a
second, low-$\alpha$ sequence as is observed in the MW.
