I got a little desktop editor written in java called lxmed (lx menu editor) https://lxmed.sourceforge.net/. It used gksu to call the .jar file. So I set out to find gksu (I have built it before). To make a very, long, story short, I found a ppa with gksu because everyone has deprecated and removed it from their distributions. Well, the work-arounds don’t fix the old scripts that use it. I didn’t add the ppa (there were many problems with that, but I used the packages). Here is the page: https://launchpad.net/~mark-pcnetspec/+archive/ubuntu/gksu/+packages
I used the GDebi Package Installer to install both of them. You need both. I’m running lubuntu 18.10 on a old MacBookAir. I hope this helps.