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
You could try this version, however this is definitely not recommended:
cat << EOF | sudo tee /etc/apt/sources.list.d/artful.list
deb http://archive.ubuntu.com/ubuntu/ artful universe
EOF
After adding the artful repository to sources, it is required to update the sources! It should be possible now to install the package...
sudo apt-get update
sudo apt-get install gcj-jdk
For other packages and their details visit the Ubuntu Package Search! Let me know if it works...
EDIT For a more detailed support you could visit the Ask Ubuntu community on StackExchange.