gksu has been removed from Ubuntu 18.04
Ubuntu 18.04 says goodbye to gksu as it is no longer installed by default. Actually it can't be installed at all because it no longer has an installation candidate. Searching in synaptic returns zero results for gksu and opening a terminal and using apt gives the following output.
Reading package lists... Done Building dependency tree Reading state information... Done Package gksu is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source E: Package 'gksu' has no installation candidate
Do a search on duck and you will find that gksu has not only been deprecated, but has already been removed from Debian. Ubuntu is just following. Developers have suggested that people should start looking towards using PolicyKit. For those of us still running and using older versions of Ubuntu, we can continue using gksu until we upgrade.