Redhat & CentOS Setting Display Manager Default:

How to set the default display manager and configure it for remote XDMCP desktop display.

Display Manager Default:

The default display manager for Red Had based systems can be set in /etc/sysconfig/desktop 
Desktop choices: GNOME, KDE 
Display Manager choices: GNOME, XDM, KDE

Example /etc/sysconfig/desktop

DESKTOP="GNOME"
DISPLAYMANAGER="GNOME"

Specifying GNOME as a display manager will invoke GDM.

Upon boot, the system will run /etc/init/prefdm.conf which will in turn call the "Preferred Display Manager" (prefdm) script /etc/X11/prefdmwhich will utilize the settings in /etc/sysconfig/desktop if available.