Because I manually installed Gnome, sudo, and disabled the root password after installing Debian, I was unable to use Gnome's administration applications. When I ran anything under the Administration menu, I would get a prompt: "Enter the administrative password".

The solution was to change the gconf key /apps/gksu/sudo-mode to true. I changed this in my own gconf DB, not the system DB, but either place should work.

According to Debian bug 57070, sudo-mode will be set automatically if Gnome is installed when the Debian installer is run.

Update

The user-setup-apply script in the user-setup package is responsible for setting up gksu. (See bug 481689.) Here are equivalent commands that can be run after the system is already installed.

sudo update-alternatives --set libgksu-gconf-defaults /usr/share/libgksu/debian/gconf-defaults.libgksu-sudo
sudo update-gconf-defaults