il try to give u a simple answer. hope it helps.
the gain knob is used to allow the amount of power that will be fed to the speakers/subs. the gain settings on different amps will vary depending on the rated power they produce per channel.
how to you set the gain? its pretty simple. before you start setting the knobs on the amp, turn them all to zero. this includes the gain. Remember, the gains need to be set differently for speakers and subs (in case of a 4 channel amp). Turn the volume on the headunit to 75% (remember, eqs should be flat). after this, slowly start turning the gain knob clockwise. at the point where you start hearing distortion from the speakers, you need to stop. This is the limit of the speakers, feeding more power to them via the gain knob will burn them. As a precaution, slightly turn the knob counterclockwise.
how this helps? well, basically if you decide to listen to your speakers at very high volumes the next time (to showoff to ur girlfriend ), rest assured, you will know you wont end up burning those expensive speakers.... and maybe lose more than just those lol
hope this helped