any noise is probably mostly from your amp or guitar not the gmajor, but the gmajor noise gate will help fix that..
the in/out knobs will be different for everyones setup..
basically you select between consumer and pro input ranges, one is more sensitive to input than the other, then setup your amp volume and input knob volume kinda at the same time to achieve the input levels reccomended in the manual..
amp volume can be whatever you think best for driving the amp nicely, then the input knob to adjust input levels. don't push the input knob full bore, it may create noise(guessing) in which case change to pro input range and try it then or maybe it was consumer? can't remember..
then for your output just whatever you want to play at. think of this as your new master volume control if you have a regular setup. don't adjust volume on the amp anymore if your input levels are perfect enough..
you can change the output range in i/o settings to make that knob dial in REALLY loud or really quiet volumes.
hope it helps, sorry if you already know most of this, it's just a recap cos you asked about the knobs..