When you set the gain on your amp you are supposed to disconnect all the speakers and play a test tone within the amps range(50hz for a sub amp is what i use). Set your volume on the head unit to 75% and turn off any bass boost, the EQ to flat and set the sub level to 0. Then you calculate what voltage the speaker terminals should be producing and use a DMM to read it. You adjust the gain until it is the correct voltage. I dont have a link to a calculator to figure out what voltage you need but im sure someone else here will have one.
v = sqrt (pwr * R) if you're looking to get 1000w at 1 ohm your target voltage is sqrt(1000) or 31.6v. IMO - since the amp is rated at 14.5V I would suggest using 900w or 30v.