Hi Renan,
There are several reasons why you are receiving the gdm error message, and here are several ways to fix this problem.
Problem 1. Your /tmp might not be writable.
a. Upon bootup, press ctrl+alt+f1, then login as root.
b. Then execute 'chmod 777 -R /tmp'
c. Restart your gdm server by running '/etc/rc.d/gdm restart'
Problem 2. GDM could not start because there are files in /tmp/ from previous sessions that it try to use.
a. Upon bootup, press ctrl+alt+f1, then login as root.
b. Then execute 'rm -rf /tmp/*'
c. Restart your gdm server by running '/etc/rc.d/gdm restart'
Problem 3. GDM is trying to run some accessibility tools that you might have uninstalled or not working.
a. Upon bootup, press ctrl+alt+f1, then login as root.
b. Then execute 'nano /usr/share/gdm/defaults.conf'
c. Comment out the line GtkModulesList=... with #GtkModulesList=...
d. Restart your gdm server by running '/etc/rc.d/gdm restart'