Edit the gdm default file:
Look at the end of the file for:
sudo gedit /etc/gdm/Init/Default
Add the touch command:
Reboot your computer and look for the /var/tmp/testgdm if it exist:
If the file doesn't exist, then the gdm default is not executed, meaning the code you have added from the first post is also not executed.
ls -alg /var/tmp/testgdm
Pls note this is to verify the file /etc/gdm/Init/Default is executed, so the coded provided in the first post is also executed.