The gas-phase metallicity distribution in galaxies offers significant information on their evolution. We report the discovery of negative radial gradients in the gas-phase metallicity of the narrow-line region (NLR) of the nine galaxies in the Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM) galaxies: Centaurus A, Circinus, IC 5063, NGC 1068, NGC 1365, NGC 1386, NGC 2992, NGC 4945, and NGC 5643. From the strong-line abundance relations for active galactic nuclei (AGNs) and star-forming regions, along with emission-line ratio diagnostics, we were able to determine spatially resolved gas-phase metallicities for the kinematic components, galaxy disc, and outflow. These relations involve sensitive strong emission lines, specifically [O III]λ5007, [N II]λ6584, Hα, Hβ, [S II]λ6716, and [S II]λ6731. The existence of predominantly negative radial metallicity gradients in these AGN host galaxies indicates that metals are not necessarily moved from the central regions to the outskirts by AGN activity. We find that the gas-phase metallicity in galaxies could follow the general inside-out star formation scenario.