Amplifier gain is a sensitivity adjustment, not a volume control — its job is to match your amplifier's input stage to the output voltage of your head unit, not to make the system louder. Treating gain as a volume knob is one of the most common car-audio setup mistakes.
The standard approach: set your head unit's volume to roughly 75–80% of maximum (just below the point where its own output starts to distort), then adjust the amplifier's gain upward until the system reaches your desired loudness without audible distortion or clipping at that fixed head unit volume.
A multimeter or an oscilloscope-based gain-setting tool measuring actual clipping at the amp's speaker output is far more accurate than setting gain by ear, especially for subwoofer channels where distortion can be hard to hear under bass content.
Once gain is set correctly, leave the head unit volume as your only day-to-day control. If you find yourself needing to push the head unit volume to its very top just to get adequate loudness, that's usually a sign gain needs to be revisited — not that you need a louder amplifier.