Gamecube controller (or DDR Mat) to USB MIDI. Install "Nintendo" and "MIDIUSB" in arduino library-manager. Use this on a programmable-USB arduino, like Leonardo.

gpc_midi2.ino

#include "Nintendo.h"
#include "MIDIUSB.h"

// attach gamecube controller red wire on this digital pin
#define GC_PIN 7

// MIDI controls for each pad
int ctrls[6] = { 10, 11, 12, 13, 14, 15 };

// track previous state
boolean buttonsPrevious[6] = { true, true,  true, true, true, true };

CGamecubeController GamecubeController(GC_PIN);

void controlChange(byte channel, byte control, byte value) {
  midiEventPacket_t event = {0x0B, 0xB0 | channel, control, value};
  MidiUSB.sendMIDI(event);
}

void setup() {}

void loop() {
  auto report = GamecubeController.getReport();

  boolean buttonsNow[6] = {
    report.dup,
    report.ddown,
    report.dleft,
    report.dright,
    report.a,
    report.b
  };
   
  for (int i = 0; i < 6; i++) {
    if (buttonsPrevious[i] != buttonsNow[i]){
      controlChange(1, ctrls[i], buttonsNow[i] ? 127 : 0);
    }
    buttonsPrevious[i] = buttonsNow[i];
  }
}