The Output Gain function is deep within the Function menu, so you'll need to press the CATEGORY >> or << button several times to reach the Output Gain function.
If there is a button on the panel controls that jumps directly to a specific function in the Function menu, you can sometimes use that as a way to reach some desired function more quickly.
For instance, if you press and hold the HARMONY ON/OFF button it will take you to the Harmony Type function, which is close to the Output Gain function in the Function menu, as you can see on page 74.
Therefore you can press and hold the HARMONY ON/OFF button, then when the Harmony Type function appears you should be able to press the CATEGORY << button three (3) times to reach the Output Gain function:
(1) Press and hold the HARMONY ON/OFF button. The Harmony Type function should appear.
(2) Press the CATEGORY << button. The I. A. Control Depth function should appear.
(3) Press the CATEGORY << button. The I. A. Control function should appear.
(4) Press the CATEGORY << button. The Output Gain function should appear.