GDM monitor configuration
Topics: GDM
GDM can use a user’s GNOME monitor configuration as its default display layout.
Subtopics
Setting the monitor configuration for GDM
GDM uses the same configuration file format as GNOME. You can have it simply copy a user’s current configuration at startup.
Create /etc/systemd/system/gdm.service.d/override.conf:
[Service]
ExecStartPre=/bin/cp /home/_user_/.config/monitors.xml /var/lib/gdm/.config/monitors.xml
See GDM - ArchWiki for details.