ohh listen ... i just noticed that ur using gedit .. so u will have to login as root ..
i mean on login screen .. when u restart .. before ur classic error hehe
provide username as root
and provide password
enter and edit the file with gedit
if u just wanna use su .. u will have to use text editor in the same virtual terminal .. in which u used su
if ur new .. then VI is headache .. so login as proper root through graphical login screen