It's a miracle he's unhurt! That must have been horrifying. We have a lab and a pit that have SSA that have to be kept apart at all times or there WILL be a fight.... The lab starts it and of course gets the short end of the stick....
Honestly, I would not hesitate to kill another dog if it meant protecting my own. I'm glad everyone was okay. Good luck to you and Gus, and keeping my fingers crossed for you that didn't make all of the reaction training you've been doing with him moot.