Gain is the amount of gain the amplier has usually between 20 to 30 db and power is the amount of watts it will put out into an impedance in ohms. The higher the gain the more loud it will be at a certain volume setting on your preamp or amp but with the higher gain more noise will be introduced but if you have efficient speakers you will always want less gain to lower the noise and use more of your volume control. With less efficient speakers you may need the extra gain because you will run out of volume control too quickly so it is for system matching. Power is a relative thing that also depends on how much current you have in the circuit, tube amps are high voltage low current, and solid state amps are low voltage high current, so different types of speakers will like one or the other.
