The file can only be written to by a user with superuser privileges. So, you need to use sudo:
Either in a terminal or using the Run Application dialog (ALT+F2), type:
gksu gedit /etc/X11/xorg.conf
The file can only be written to by a user with superuser privileges. So, you need to use sudo:
Either in a terminal or using the Run Application dialog (ALT+F2), type:
gksu gedit /etc/X11/xorg.conf