Well unless someone else can chime in, if I were you, I would try to set it by ear.
Connect your speakers, put your amp level knob at the lowest setting, headunit to 80% of max.
Play the 1kHz tone on repeat, and slowly bump up the level until you can hear a pitch shift (clipping) or distortion. Readjust the amp level so it is slightly below clipping/distorting level.
Then play some of your music. If it sounds distortion free, I think you have successfully set your gain.