We determined precise and homogeneous metallicities for old open clusters (>=500Myr) from all-sky catalogues of stellar parameters leveraging Gaia spectrophotometry. Our purpose was to revisit the metallicity distribution of the oldest open clusters as a function of their Galactic position and age with improved statistics. Several catalogues of stellar parameters have been cross-matched to the most recent census of open clusters and their members. Median metallicities per cluster and per catalogue were evaluated by comparison to high-resolution spectroscopy. Our sample includes 1113 open clusters. A typical precision of 0.05dex is achieved for a subsample of nearly 600 clusters when only their bright red giants are considered. Metal-rich and metal-poor clusters never studied before are presented, as well as remnants of dissolving clusters. Galactic maps show a smooth decrease of metallicity from inside to outside the disc. We measured the radial and vertical gradients, their evolution with time and the age-metallicity relation in bins of Galactocentric distance.