How to GDM login-screen resolution

UHD-Monitor. Gnome (Manjaro Testing). Radeon-RX5500.
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The login-screen is in full UHD-Resolution (3840 x 2160)
I want: 1280 x 800.
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This does not work (Arch GDM wiki ):

Change Desktop screen to 1280 x 800.
sudo cp -i .config/monitors.xml /var/lib/gdm/.config/
chown gdm:gdm /var/lib/gdm/.config/monitors.xml