Context. The bimodality of the Milky Way disc, in the form of a thick short disc and a thinner more radially extended one, encrypts the complex internal evolution of our Galaxy and its interaction with the environment.Aims. To disentangle the different competing physical processes at play in Galactic evolution, a detailed chrono-chemical-kinematical and dynamical characterisation of the disc bimodality is necessary, including high number statistics.Methods. Here, we make use of an extremely precise sub-sample of the Gaia DR3 GSP-Spec catalogue of stellar chemo-physical 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(ɡ) and [M/H], respectively.Results. The stellar parameter precision allows us to break the age–metallicity degeneracy of disc stars. For the first time, the disc bimodality in the Kiel diagram 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 us to demonstrate that an age gap is needed to explain the evolutionary sequence separation, in agreement with previous age–metallicity relations obtained using sub-giant stars. A bimodal distribution in the stellar mass-[α/Fe] plane is observed at constant metallicity. Finally, a selection of stars with [M/H]=0.45±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 of 5–13 Gyr, in agreement with previous literature findings. 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.Conclusions. The Gaia GSP-Spec Kiel diagrams 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.