Unable to edit /etc/apt/sources.list file

Use the terminal text editor program nano instead of gedit to edit /etc/apt/sources.list. Open the terminal and type:

sudo nano /etc/apt/sources.list

The instructions for using nano are always displayed at the bottom of the page. Use the keyboard combination Ctrl + O and after that press Enter to save the file to its current location. Use the keyboard combination Ctrl + X to exit nano.

You can also use the terminal program vim to edit text files, but nano is easier to use.

gksu is probably set to use su instead of sudo as the authentication method. Therefore, it's asking you for the administrative (root) password.

To change this, run gksu-properties and choose sudo as the Authentication Method. This way, gksu will ask you for your password.

You'll notice that the dialog box that asks you for the password changes from:

which is asking for the administrative (root) password to:

which asks for your password to perform administrative tasks.