This seems like it would be practical, but wouldn't I still need the ground pin? It says to move it to the power pin right?
The trick is to use the cable shield as ground, since the connector shield is connected to ground inside the Gameboy. Alternatively, you could move and reuse the SOUT pin (the one that sends data from the Gameboy) as +5V if you don't need that functionality (sending MIDI from the Gameboy.) Without some kind of ground connection, the circuit won't work.
You lost me at cable shield and connector shield. Are you referring to certain pins? I definitely need to use the SOUT pin. I need all the pins necessary to use full midi in and out capabilities.