Why is gksu asking me for root password and not my password?

On one of my systems, I'm running Gentoo Linux and I've installed x11-libs/gksu-2.0.2-r1. I've disabled the root account using sudo passwd -dl root and I want to be able to run GUI apps which need root privileges using sudo. I've set gksu to use sudo for authentication using gksu-properties, but it's still asking me for *root* password when I launch an application that needs root privileges and not *mine*. What am I missing?