Not sure what system you’re using (iOS, iPadOS, Mac etc) but as far as I know there is no ‘master gain’ as such…each channel has its own gain control and if you’re using Auto Gain this may be to the left (volume reduced), central or up (no change to volume) or to the right (volume increased).
If Auto Gain alters a track’s volume you can move that channel’s gain control to suit what you want.
Also, the position of the gain knob is in different places dependant on which deck configuration you have (classic, 2 deck, 4 deck etc) and also if the iPad / iPhone is in portrait or landscape but regardless of which you have it’s located in the same space as deck volume, EQ/NM, hp/lp filter and is the smallest knob that isn’t labelled.
You might have already known all of the above but that is the gain control system and there is no overall gain control.
Unless someone knows differently…