Research and Development
Go into Settings > Devices > Displays and configure your monitors the way you want for your login screen. Click the "Save" button when done.
Copy your user's monitors.xml file into the home folder for gdm user.
To copy the monitors.xml file, open a terminal and perform the following:
sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml
sudo chown gdm:gdm ~gdm/.config/monitors.xml
Only if needed, uncomment WaylandEnable=false in the /etc/gdm3/custom.conf.