We used the ugr magnitudes of 1437467 F-G type main-sequence stars with metal abundance -2 <= [Fe/H] <= +0.2 dex and estimated radial and vertical metallicity gradients for high Galactic-latitude fields, 50 degrees < b <= 90 degrees and 0 degrees < l <= 360 degrees, of the Milky Way Galaxy. The radial metallicity gradient d[Fe/H]/dR = -0.042 +/- 0.011 dex kpc(-1) estimated for the stars with 1.31 < z <= 1.74 kpc is attributed to the thin-disc population. While, the radial gradients evaluated for stars at higher vertical distances are close to zero indicating that the thick disc and halo have not undergone a radial collapse phase at least at high Galactic latitudes. The vertical metallicity gradients estimated for stars with three different Galactic latitudes, 50 degrees < b <= 65 degrees, 65 degrees < b <= 80 degrees and 80 degrees < b <= 90 degrees do not show a strong indication for Galactic latitude dependence of our gradients. The thin disc, 0.5 < z <= 2 kpc, with a vertical metallicity gradient d[Fe/H]/dz = -0.308 +/- 0.018 dex kpc(-1), is dominant only in galactocentric distance interval 6 < R <= 10 kpc, while the thick disc (2 < z <= 5 kpc) could be observed in the intervals 6 < R <= 10 and 10 < R <= 15 kpc with compatible vertical metallicity gradients, i.e. d[Fe/H]/dz = -0.164 +/- 0.014 dex kpc(-1) and d[Fe/H]/dz = -0.172 +/- 0.016 dex kpc(-1). Five vertical metallicity gradients are estimated for the halo (z > 5 kpc) in three galactocentric distance intervals, 6 < R <= 10, 10 < R <= 15 and 15 < R <= 20 kpc. The first one corresponding to the interval 6 < R <= 10 kpc is equal to d[Fe/H]/dz = -0.023 +/- 0.006 dex kpc(-1), while the others at larger galactocentric distances are close to zero. We derived synthetic vertical metallicity gradients for 2,230,167 stars and compared them with the observed ones. There is a good agreement between the two sets of vertical metallicity gradients for the thin disc, while they are different for the thick disc. For the halo, the conspicuous difference corresponds to the galactocentric distance interval 6 < R <= 10 kpc, while they are compatible at higher galactocentric distance intervals (C) 2018 COSPAR. Published by Elsevier Ltd. All rights reserved.