Gain on an amp is how sensitive the amp is to the input signal. Say that turning the gain all the way counter-clockwise (down) is 5volts and all the way clockwise (up) is .2volts. If your deck is putting out 5v, the amp then doesn't have to be nearly as sensitive to correctly process the input signal from your deck, so you turn the gain down. If the deck puts out only .2 volts, the gain has to be all the way up in order to be sensitive enough to process everything correctly.
So if your talking about output voltage FROM a deck, 5 is higher. If you're talking about input voltage TO an amp, then .2volts would be a higher sensitivity.
