Gains on amps are not listed in dB in the car audio world. They are listed in voltage. That control is there to match the output voltage of your input source (most typically your headunit) to the input of the amplifier.
It is absolutely not a volume control!!!
Also, did you try reading over here:
http://www.eatel.net/~amptech/elecdisc/
WiLL - How's that IDQ BTW?