From https://wiki.archlinux.org/title/GDM#Setup_default_monitor_settings
Some desktop environments store display settings in ~/.config/monitors.xml.
xrandr commands are then generated on the base of the file content.
GDM has a similar file stored in /var/lib/gdm/.config/monitors.xml.
If you have your monitors setup as you like (resolution, refresh rate, orientation, scaling, primary and so on) in ~/.config/monitors.xml and want GDM to honor those settings:
# cp ~/.config/monitors.xml /var/lib/gdm/.config
To automatically re-configure the monitor setup on each boot, use a drop-in file for gdm.service:
/etc/systemd/system/gdm.service/override.conf[Service] ExecStartPre=/bin/cp /home/user/.config/monitors.xml /var/lib/gdm/.config/monitors.xml