Gain volume in the mixer is one volume below the input sections of each channel. The job of this volume is to amplify the sound and adjust the amount of input signals from each of the sound rows to the mixer.
Under the input jacks in mixers, there are usually several volumes and potentiometers. Usually the first lean is the gain control. In Dynacord mixers like the PowerMate 2200-3, after the gain volume, there are dynamic equalization volumes.