  We use the calibrations by Calamida et al. and by Hilker et al., and the
standardised synthetic photometry in the v, b, and y Stromgren passbands from
Gaia DR3 BP/RP spectra, to obtain photometric metallicities for a selected
sample of 694233 old Galactic giant stars having |b|>20.0 and parallax
uncertainties lower than 10%. The zero point of both sets of photometric
metallicities has been shifted to to ensure optimal match with the
spectroscopic [Fe/H] values for 44785 stars in common with APOGEE DR17,
focusing on the metallicity range where they provide the highest accuracy. The
metallicities derived in this way from the Calamida et al. calibration display
a typical accuracy of ~0.1 dex and 1 sigma precision ~0.2 dex in the range -2.2
<=[Fe/H]<= -0.4, while they show a systematic trend with [Fe/H] at higher
metallicity, beyond the applicability range of the relation. Those derived from
the Hilker et al. calibration display, in general, worse precision, and lower
accuracy in the metal-poor regime, but have a median accuracy < 0.05 dex for
[Fe/H]>= -0.8. These results are confirmed and, consequently, the metallicities
validated, by comparison with large sets of spectroscopic metallicities from
various surveys. The newly obtained metallicities are used to derive
metallicity distributions for several previously identified sub-structures in
the Galactic halo with an unprecedented number of stars. The catalogue
including both sets of metallicities and the associated uncertainties is made
publicly available.
