Gunner did good and the other dog was in his space growling from what i gather. Yeah, maybe gunner jumped the gun a bit, but hey he was in a down being a good boy, undoubtedly he felt something not right and warned the other dog to back off, he did leave it and obey you. That's a well trained dog, sounds to me. Please don't muzzle him. Most i would do is keep him leashed to me on the slow walks so as also to see how he will do after this episode.