Yes, it is ok. The gains on the amp are like a volume knob. They control front and rear outputs independently.
Please never use this phrase ever again. As it is misleading and false.
Op you can use a more powerful amp than is required.
The gain knob on an amp allows you to increase the amps output until it matches the input of the headunits signal. Beyond the input signal you gain nothing but distortion. But you you keep it below the input signal you can throttle the amps output and in turn run less power than the amps rated output.
This is beneficial as it allows for headroom on dynamic music.
The gain knob can control the amps output wattage but only up to the point in which it matches the input signal. Anything below that is fine. Anything above is clipping.