Boss GT-PRO Owner's Manual page 140

Driver installation and settings (Macintosh)
5
In Choose a source for sound input (Device), select USB audio.
fig.12-0460
* Do not check Play sound through output device.
6
When you are finished making settings, close the Sound dialog box.
7
From the File menu, select Quit.
140
If USB audio is not displayed,
close the Sound dialog box, and
disconnect the GT-PRO's USB
cable from the Macintosh.
Perform the driver installation (p.
138) once again.