When sending MIDI messages to/from the GT-100 through USB, you'll want to set the MIDI In Select mode to "USB Auto." Doing so allows the GT-100's USB port to transmit/receive MIDI messages. Here's how to select it:
1. Press the SYSTEM button.
2. Turn knob 4 to highlight/select the "MIDI SETTINGS" icon.
3. Press PAGE right to select tab 2.
4. Turn knob 6 counterclockwise to choose "USB(AUTO)."
5. Press EXIT when you're finished. The new setting is stored automatically.