Why GUI is not working in Red Hat Enterprise Linux 6 ?

Issue

• Installed gnome-session, xterm, gdm and still can not open a GUI window.

• Running gdm says this:

[root@xxx ~]# gdm
gdm-binary[17119]: WARNING: GdmDisplay: display lasted 0.012755 seconds
gdm-binary[17119]: WARNING: GdmDisplay: display lasted 0.012482 seconds
gdm-binary[17119]: WARNING: GdmDisplay: display lasted 0.012475 seconds
gdm-binary[17119]: WARNING: GdmDisplay: display lasted 0.012439 seconds
gdm-binary[17119]: WARNING: GdmDisplay: display lasted 0.012584 seconds
gdm-binary[17119]: WARNING: GdmDisplay: display lasted 0.012448 seconds
gdm-binary[17119]: WARNING: GdmLocalDisplayFactory: maximum number of X display failures reached: check X server log for errors

• Messages says it can't find /usr/bin/Xorg:

gdm-simple-slave[xxxx]: WARNING: Could not start command '/usr/bin/Xorg -br -verbose -audit 4 ': Failed to execute child process "/usr/bin/Xorg" (No such file or directory)