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GDM is the GNOME display manager (a graphical login program) for the windowing systems X11 and Wayland.
In some cases it is better and more easy to use GDM if you are running GNOME.
As it is a fact that GNOME is builded to use GDM so most of the common powersaving and lock options are working without problems under GDM, but can cause some problems if you are using the default Antergos Loginmanager Lightdm+lightdm-webkit2-greeter. This is an issue mainly for mobile devices.
Install GDM as the default greeter:
sudo pacman -S gdm
enable GDM and disable lightdm in one step:
sudo systemctl -f enable gdm