Amplifier gain is fixed, never variable. In reality what your "gain reduction" is limiting is the amplifier input level. In other words more like "maximum volume lock". What this does will depend on how it works. The vast majority of volume controls are really attenuators. If smooth then a variable attenuator or trim pot. If stepped then a stepped attenuator with a lot of different value resistors.
Either way, the max is limited by setting a minimum amount of attenuation. In other words like beyond a certain point turning the volume up no longer turns the volume up. It doesn't limit the amp. It limits the input to the amp.