[Pisces]

I don't have the game but from a technical point of view: Low and High pass are filters. Low pass means frequencies up to the dial setting are audible. All above will be diminished stronger the higher they are. Similarly high pass only allows frequencies above the dial setting to be audible. The lower below the setting the less strong they will sound.

I (edit: don't) know why they have 2 different dials for gain and volume. As they both would imply an amplification of sorts. Maybe, if they implemented it, gain is amplification for some kind of detector and the volume dial is amplification for your headphones. But I really should let more knowledgable player answer this.