Until today, you can download soundbanks for the Java Sound API from Oracle here. What surprises me is, that they are not hidden on an old subpage -- you find them freely accessible on their website and they pop up very high in the search results.
According to https://stackoverflow.com/a/31237701/16394149 the support for this GM soundbank file format was dropped with Java 7. Now I'm really really curious about these files and indeed I can read them in an old XP vm with Java 1.3. Oracle still provides a working demo app here. They even provide the source code for the Java 1.3 SDK. Unfortunately, the javax folder does not contain the source for the sound API.
I can't find anything regarding this .gm file format. Nothing. No specification from The MIDI Association, no documentation, not implementation. Also, the source code of Tritonus (the open source implementation of Java Sound) does not contain any source to read these files.
Does anybody knows how to work with these files?
SINEare not so hard to guess, so it may be possible to reverse engineer the format if you spend some time on it. I'd also guess the source code for the actual implementation was never released. If someone finds on old binary Java SDK/RE, a Java decompiler may help.