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

I'm having trouble getting the GDM login screen to show up when I
connect from a remote host.  I'm using Xming on the local Windows
machine, but every time I connect to the remote server all I get is a
gray window with the X cursor.  I have "Enable=true" in the [xdmcp]
section of /etc/gdm/custom.conf.  IPtables is disabled.  I've googled
around and read through the xdmcp page on tldp.org but have not come
up with anything.  Can anyone help with this issue?