As you can see at http://euscan.iksaif.net/package/x11-libs/gksu/, euscan thinks a 2.0.12 version is available, but it's not Thanks for fixing it Reproducible: Always
Then what is http://people.debian.org/~kov/gksu/libgksu-2.0.12.tar.gz ?
It's the library, not the frontend ;)
Fixed