I get the same error as dweeberkitty in VL 5.9 STD.
It seems that the "gksu | kdesu" line in slack-required doesn't work here.
In my system, I have kdesu. I think it might just be the default in a fresh install. It might also be a consequence of having installed kdebase.
In any case, there is no kdesu package anywhere in the repositories. This makes gslapt try to install gksu instead (even though kdesu exists, but not as a package), and the dependency error is something wrong in the slack-required of the gksu package.
(To make things more complicated, I also have something in /usr/sbin/gksu, which is a script that calls "/sbin/vsuper", which in turn calls kdesu.)