Gksu has been deprecated from Debian and Ubuntu (specifically Ubuntu 18.04LTS, slated for release in a day or two). In a stock Ubuntu 18.04LTS install, the "gksu" package is no longer available from standard repositories, and Guymager's desktop launch icon, which issues the command "gksudo guymager" fails to launch the guymager application. In testing with the latest daily build of Ubuntu 18.04LTS, Guymager still launches ok from the command line with "sudo guymager".
Apologies - it looks like I had some sort of different (older or customized) desktop file. I see the current one uses pkexec.
Yep! Mika from Debian warned me about it some weeks ago and so I switched to pkexec. I'm glad that the gksudo dependency (which always caused problems) is over.