Comment by TazeTSchnitzel

Windows and macOS have both bundled similar low-quality licensed versions of a Roland GS sample set and low-quality software synthesisers since the late 1990's. Neither of them are as good as even the lowest-end dedicated hardware General MIDI synth from Roland was, but they're still better than nothing. You're unlucky because you happen to be using Linux. unfortunately the best in that field was and remains proprietary so this will probably never be solved on that platform.