  To disentangle the different competing physical processes at play in Galactic
evolution, a detailed chrono chemicalkinematical, and dynamical
characterisation of the disc bimodality is necessary, including high number
statistics. Here we make use of an extremely precise subsample of the Gaia DR3
GSP-Spec catalogue of stellar chemophysical parameters. The selected database
is composed of 408 800 stars with a median uncertainty of 10 K, 0.03 and 0.01
dex in Teff , log(g) and [M/H], respectively. The stellar parameter precision
allows to break the age-metallicity degeneracy of disc stars. For the first
time, the disc bimodality in the Kiel diagramme of giant stars is observed,
getting rid of interstellar absortion issues. This bimodality produces double
Red Giant Branch sequences and Red Clump features for mono-metallicity
populations. A comparison with BaSTI isochrones allows to demonstrate that an
age gap is needed to explain the evolutionary sequences separation, in
agreement with previous age-metallicity relations obtained using Main-Sequence
Turn Off stars. A bimodal distribution in the stellar mass-[alpha/Fe] plane is
observed at constant metallicity. Finally, a selection of stars with [M/H]=0.45
\pm 0.03 dex shows that the most metal-rich population in the Milky Way disc
presents an important proportion of stars with ages in the range 5-13 Gyr. This
old, extremely metal-rich population is possibly a mix of migrated stars from
the internal Galactic regions, and old disc stars formed before the last major
merger of the Milky Way. The Gaia GSP-Spec Kiel diagrammes of disc
mono-abundance stellar populations reveal a complex, non linear age-metallicity
relation crafted by internal and external processes of Galactic evolution.
Their detailed analysis opens new opportunities to reconstruct the puzzle of
the Milky Way disc bimodality.
