HOWTO: fix gksu not accepting your password

Sometimes you can get into a situation when gksu (or any application dependent on this GTK+ su frontend) doesn't accept your password (while gksudo may work OK).

It can be caused by incorrect gksu settings; to correct the situation:

1. execute gksu-properties (without any arguments), so the "Privilege granting preferences" dialog window should pop up;
2. select the correct authentication mode from the drop-down list:
• su, if the system has an active root user with password;
• sudo, if there's no active root user, and the superuser privileges are granted to a normal user through the use of sudo;
3. close the settings window;
4. finally, try to execute some gksu-dependent stuff to test it.