First things first; I am totally aware that development of GCJ was discontinued in 2017 before the release of Ubuntu 18.04, but it's still a great tool, and if anyone knows how I might be able to use it on Ubuntu 18.04, that'd be great.
I'm also aware of all the alternatives, i.e. launch4j etc, but all of the ones I have encountered have only been 'wrappers', and don't actually create machine code