GSM Z80 is the Z80 version, it works correctly, but can only output 100Hz, so
it's practically useless.  Please note, there's no bank switching, the bank
is set once, and that's it, it'll continue and wrap to beginning of Z80 RAM.
Since it wasn't practical, I never programmed a bank handler.

GSM 68k is the 68k version, the 68k does the calculation of a block, every frame.
The Z80 is a basic PCM playback driver, every V-blank it'll play a 160 buffer of
samples, it's timed approximately such; finishing 160 samples and exiting V-blank
it'll very soon interrupt back into V-blank to play the next buffer.  The 68k isn't
able to decode an entire block in a single V-blank it seems, too many multiply
instructions, so it always misses.  The output ends up with the Z80 repeating the
same two/three/four buffers before the 68k can update a buffer fully.  I'm assuming
the rate is approximately 2000Hz.