The idea is that you need your Auto Attack (which feels like it's ~80 potency) to outweigh the loss from dropping GB. GB is a 30% gain.

Hot Shot = 120 => 36 / 140 => 42
Split Shot = 140 => 42 / 160 => 48
Slug Shot = 180 => 54 / 200 => 60
Clean Shot = 200 => 60 / 220 => 66
Lead Shot = 490 => 147 / 510 => 153

That's the gain you get from GB - aka the loss you suffer when dropping it and hitting said GCD. The oGCD stuff is a bit wonky because you can only use two of your oGCD hits without being under GB - Blank and Head Graze/Suppressive Fire. Both are 100 Potency, so a gain of 30 with GB equipped. Applying these to the above, the only situation where you could GCD > oGCD without GB and still get a gain is if you're hitting Hot Shot or Split Shot - and even then the gain is minimal at best and ONLY a gain IF there's actually an Auto in that timeframe.

The general rule of thumb is you want 1 auto per GCD you use out of GB.

If you end up delaying Ricochet or Gauss Round by turning GB off, you're taking significant hits to damage - those things are 260 and 390 potency off-gcd attacks - comparable with Dragoon's Jump and PS+Jump - two of our strongest attacks. Don't delay them if you can help it.

This is all garnered from a bit of research into Aiurily's videos as well as my own number crunching as seen here. I call Autos 80 potency because they hit about as hard as Rook attacks, which are 80 potency.