“gain” in the sense of distortion, a.k.a. Drive in the Amp block? If so, that’s very different from volumes, of which there are three: Amp/Tone Volume, the combined Dual Tone volume on the Home Page, and Master Volume.
Changing the amount of Drive changes the timbre of the patch. You can do that if you want, but you’re deviating from the intentions of the person who made it. Which is fine if you like it better that way. And why would they use what you consider a low level of distortion? Specifically because, as I first said, that was the sound they wanted, for example if they felt it better simulated the tone of a particular artist. Sorry, but this seems obvious. Again, if your preferences differ, do whatever you like. It doesn’t mean there was no reason for using a level of Drive less than 8. Plenty of Tones/Amps call for that.
On the other hand, changing any of the volumes just makes the patch louder. And, unless you have a specific reason to turn them down at all—chiefly to ensure certain Tones don’t come out a lot louder than others live—having the Tone and Master Volumes as high as they will go without clipping is optimal because that maximises the signal-to-noise ratio of the POD. The manual explains this clearly.