Hi Philip,
GPIO outputs refer to digital output pins used to control external LEDs and such. The audio DAC outputs are called simply outputs, as shown below:
The most frequent cause for on-screen controls not working is a zoom level not exactly 100%. Try pressing Shift-Ctrl-F6 to restore normal size, then see if your controls work -- see http://ez.analog.com/docs/DOC-1649
Best regards,
Bob