GES vs. Gaia metallicity (uncalibrated in the upper panel, calibrated in the central panel and OC-calibrated in the bottom panel) for 52 open clusters in common: 34 clusters with 3 or more members for both spectroscopic surveys are shown in magenta, while the 18 clusters with less than 3 members in one of the two surveys are in light grey. Left: Gaia vs. GES average cluster metallicities; centre: metallicity distributions of the clusters as seen in GES (red) and in Gaia (blue); right: distribution of the averaged-metallicity differences Δ[Fe/H] = [Fe/H]_Gaia − [Fe/H]_GES.