As princec say, OpenJDK can be used for commercial games without paying for any license.
The workaround I've been using for development: in Eclipse I forego modules, in which case (if I understand correctly) the project is treated as a single module that makes use of all of Java modules.
Then, you can still make a .jar, and then follow
this tutorial for building a customized runtime using JLINK.
And Inno-Setup 6 (for Windows) or this
MacOSX hack can be used to make an installable.
I'm looking into revising the tutorial I wrote on command-line modular compilation, updating for JavaFX 11 (OpenJFX 11 is the LTS, and also can be used for commercial games), and will see if there is a better way than "crash-surfing" to figure out the contents of the needed module-info.java.
I assume the folks supporting/promoting LibGDX would have tutorials by now on how to use their library with the major IDEs. My first guess is that (with Eclipse) LibGDX would be added as an external jar or library in the build configuration, possible on the Module Path instead of the Class Path. But I haven't looked into this since I don't use it.