libgksu 2.0.0 is out, and they moved all the old packages to an obsolete directory, so the recently added libgksu-1.9.8 isn't able to download properly. Sorry for the bad news... 5:(
libgksu-1.9.8 is here: http://people.debian.org/~kov/gksu/old_stuff/libgksu2/
Thanks, I've fixed this (and gksu too)