How can I verify that the buttons on my GiO are transmitting data?

Double-click  and open GiOconfig from the Mac's Applications folder.

Under the Expression pedal curve setting you'll see a Value: and grey rectangular bar.

When pressing a button on the GiO the rectangular bar should turn blue and a Value of 127 displayed.

This would indicate that GiO is transmitting MIDI data to the Mac and can be checked with all the buttons of the GiO.