Of course nobody really knows how it would have played out, but my guess is that if the GG didn't intervene, the BG would not have had the shotgun pointed at him. The BG would have taken the money and left. Its a gamble either way. You risk getting shot by intervening and accidentally shooting someone else in the store when you start shooting 6-7 rounds and only hitting your mark twice. Just one stray bullet in a store is no bueno.
My personal opinion is that i carry for myself. If I do not have to intervene I won't. If the gun is pointing at someone else and the perp hasn't fired any shots, i would rather have them walk out with money with zero shots fired from anyone. The risk of the BG start shooting because someone tried to be a hero is not something i am willing to take. And just because you carry doesn't mean you should get involved. He's lucky he didn't shoot anybody else.