Updated gdm packages that fix a bug are now available for Red Hat Enterprise Linux 6.
The GNOME Display Manager (GDM) is a highly configurable reimplementation of XDM, the X Display Manager. GDM allows you to log into your system with the X Window System running and supports running several different X sessions on your local machine at the same time.
When gdm was used to connect to a server via XDMCP (X Display Manager Control Protocol), another connection to a remote system using the "ssh -X" command resulted in wrong authorization with the X server. Consequently, applications such as xterm could not be displayed on the remote system. This update provides a compatible MIT-MAGIC-COOKIE-1 key in the described scenario, thus fixing this bug.
All users of gdm are advised to upgrade to these updated packages, which fix this bug.