Hi Rich,
The GB knob has a pretty broad range, so it needs to be set to a position where the detector is quiet as the coil is "pumped" at the ground. It's described in the manual, but you want to find the knob setting where the coil sounds when pulling it up and also where it sounds when lowering the coil. You set the control between these points, where the sound is quiet or neutral as you raise/lower the coil. If you have the GB control set fully "off" counterclockwise, you're actually setting the GB way negative and it probably would sound off constantly in that position.
I've found that pressing the Ground Trac button will "center" or balance the GB circuit even when the knob is off-center. This probably isn't the best way, however. For a "default" position, just try putting it at 2:00 or thereabouts. If you only use it to pinpoint, it needn't be exact and the Ground Trac will help take up the slack of a less-than-perfect GB. Some experts intentionally offset the GB for a desired result, say for tracking black sands or meteorite hunting.
As to the Hot vs. Not observation, I'm just going by some folks who reported a lot of falsing until they reduced the sensitivity way back. On mine, I can usually run it full-on, but it will sound off and false when I lift the coil up and aim it at overhead powerlines. In that spot at our local schoolyard, I only needed to dial it back to around 3:00. The manual says the Sensitivity adjustment is mainly to control EMI issues, but it also admits depth of detection is reduced at lower settings. So you normally want to run this as high as the hunting location allows to help find smaller or deeper targets.
If you like, you can find the "best" spot for your hunting sites and personal preferences and mark the position on the control panel with a little triangle of electrical tape. The tape is nice because you can change it later if your opinion on the best place for it changes.
I just seriously played with the autonotch and notch today for about the first time to see how they work. I like them, even though I almost always hunt with zero discrimination and make the call myself to dig or not. I like to hear everything, yet dig selectively, based on the junk I find at a particular location. But sometimes it's nice to just get rid of a nuisance target in a place that's really infested with a junk target of one type of another. For me, that's usually rusty nails that litter old mines. The notch button just cleanly knocked out rusty nails but kept a test nickel I laid nearby. I find a lot of spent slugs from bullets. That tells me if it would have been a nugget, I would've dug that signal.
-Ed