[CentOS] XDMCP - no GDM login, only gray screen with X

>   Set Enable=true under XDMCP section on /usr/share/gdm/defaults.conf
> and relogin it will work you out.
>

I thought about this until I read the directions in the defaults.conf file:

# This file should not be updated by hand.  Since GDM 2.13.0.4,
# configuration choices in the /etc/X11/gdm/gdm.conf-custom file will
# override the default values specified in this file (installation
# locations of the configuration files may vary for different distros).
# This defaults.conf file may be overwritten on upgrade, so to ensure
# that your configuration choices are not lost, please make sure that
# your changes are made only the the custom.conf file.

Indeed, the gdmsetup program does not touch this file, rather it edits
the custom.conf file.