I think Gunner behaved very well, is well-trained, and did everything you could expect of him!
Except that he defended himself appropriately when he felt he had to - ( my dog does, as well, and I am ok with that. My dog will meet attacking offleash dogs with a growl and a lunge, teeth to teeth, and I am on his side. )
re the other owner: if his large dog has growled at other dogs and ignores your recall sometimes, why the heck would he let him offleash?!! That is just asking for trouble. Or maybe it was the first time his dog ever growled at another dog? But I doubt it.
re Gunner: it is good to keep in mind though, that you know now...he is not the kind that will growl/bark and posture - he is the kind that may do a swift lunge and bite - so that is harder to handle than a dog that growls and gives a lot of warnings.
re Down: I think it's great that he obeys you so well! I wonder - does having him in a down with a large growling dog nearby, increase his tension? Perhaps if he were free to move (on leash) then he would have growled and circled around instead of going straight for a bite.
IF you are muzzle shopping (I guess it's good to have one around just in case) - I trained my dog to wear a basket muzzle (Baskerville) and I use it for vet visits. I like it because it's lightweight, easy to wash, he can pant, he can drink, he can eat treats. It's not leather, it's plastic - but I feel like that helps with keeping it clean and non-smelly. ( You can do things like smear peanut butter on the inside, and easily wipe it off/wash it. )
I also agree that I don't think Gunner needs a muzzle for every outing! But live and learn - you know how he is likely to react, now, so be on guard against situations that could trigger it. ( For instance, I learned (the hard way) not to let people reach out to pet my dog on the head. )
Rumo ~ rescue shepherd/husky mix