By default, the Gnome login screen chooses the monitor layout and screen resolution automatically. If you want to change this, you may have been like me and searching everywhere and finding references to xrandr and init files. None of these solutions worked for me. I finally did find the solution at The Archlinux Forums.
Changing the login screen resolution on CentOS 7 using Gnome 3 (The default desktop install) is a simple but non-obvious process.
Login and Setup Your Screen.
Login to Gnome as your normally would. Use the Settings to configure your monitor’s layout and resolution to what you would like your login page to be.
Once everything is laid out and the correct resolutions you would like, click apply. This will create a file in your home directory .config folder called
Copy the Configuration.
To set this layout at the gnome default layout (the one gnome will use for your login screen), copy it to
/var/lib/gdm/.config/. You will need to have administrator privileges.
sudo cp ~/.config/monitors.xml /var/lib/gdm/.config/monitors.xml
Thats it! Reboot and check out your login screen.