I'm not Aiurily, but according to their guide, you outlined the exact situation when you would need to use Feint. There was one specific situation that was mentioned for A7S where you may have GB off for a Hot Shot and turn it on for the Lead Shot, only to need to move for the merry-go-round. In that situation, you just Feint until GB is ready, then turn it off and auto/Split-Slug-Clean until you're good again. It's also useful in a situation where you have to move and you have ogcd attacks ready to go - better to Feint>ogcd than drop GB and do Split>ogcd, I think!

It's ALSO worth noting, though, that if you won't be strafing while turning GB off (and thereby missing your AA's), you'll get net gains by keeping it on and rolling Feint instead of Split out of GB. Dropping GB is only a gain if you don't miss AA's.