Comment 11 for bug 370607

It also works fine for me...

bryce@chideok:/etc/gdm$ sudo glxgears
Running synchronized to the vertical refresh. The framerate should be
approximately the same as the monitor refresh rate.
13834 frames in 5.0 seconds = 2766.745 FPS

I would first check gdm.conf against a stock config. I've attached my own, which is not stock but is known-good so should serve as a reference. There are settings in that file that can affect whether X applications are allowed to be run by other users.

Next I would check what options are being specified on the X binary. Here's mine:

  /usr/X11R6/bin/X :0 -br -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7

In particular -nolisten is of note if you're trying to run X apps while logged into the system non-locally. If using ssh, then the -X flag would be of use there.

Anyway, based on past experience my guess here is not that it's a bug in how X is working but rather is some stray security configuration setting.