Looks like GPA is only available for older versions of Ubuntu. Looking around for a minute, though, I found a Forum Post suggesting that you could either install an GPA from source (and there is a warning about newer versions causing some problems), or use Geany instead. I think going the Geany route might be a little better:
Also, I should point out that you can't edit /etc/apt/sources.list as you've tried. It's actually a text file that lists the repositories to search for software. If you wanted to add a repository to it, you can manuall edit the file by using:
or you can use the 'add-apt-repository' command:
sudo nano /etc/apt/sources.list # or replace 'nano' with a more preferred text editor
Note that these are all administrative tasks, and so you need 'sudo' to perform them, which grants admin level permissions for the task at hand.
sudo add-apt-repository ppa:<ppa_name>