This guide helps you choose on which screen, your loginscreen is displayed on in GDM/Fedora 25
GDM i the new era uses Wayland, therefore config file must be Wayland compatible.
Solutions is partly gui based, and partly terminal based.
Login with your user, with Wayland session, select with cog on loginscreen. NB: Before Fedora 25 you should select Gnome with Wayland, on Fedora 25 you just select Gnome.
After login, set preferred display settings from controlpanel.
Start a terminal and run the following commands:
sudo cp .config/monitors.xml /var/lib/gdm/.config/monitors.xml sudo chown gdm:gdm /var/lib/gdm/.config/monitors.xml
Restart and it should all work.
My setup, currently is a Lenovo X230 with a external VGA monitor, now with loginscreen on VGA monitor. When VGA monitor is not connected, the loginscreen shows on builtin monitor.