In RHEL7 how do I enable XDMCP?

Tried the following steps:

• Installed GDM packages using command line:
  yum install gdm -y

Edited /etc/gdm/custom.conf file and added the foolowing entries:
[xdmcp]
Enable=true

[security]
DisallowTCP=false
AllowRemoteRoot=true

• Rebooted RHEL7.

On the X server, tried XDMCP connection to RHEL7 host, got back message: "Oh no! Something has gone wrong. A problem has occurred and the system can't recover. Please log out and try again."

Thank you for your attention.