I'm not a fan of GL for the reason of the possibility of spinal injury, for one and for the other, the muzzle thing. For dogs, something around their muzzle equals (in dog language) a dominating thing. Dogs will place their mouth over another's muzzle to attempt to make the dog submit. And I'm not into that kind of force when training. (whenever possible. Obviously we need to use a leash and some force) Now one may say, "but dogs teach eachother that way, so why not us?" Well, my philosophy is that we are not dogs and they know it and I don't think we can emulate closely enough to what dogs do to eachother to make it worth all that much. We can miss certain preceding subtle body language or gestures, looks and positioning which may go with this mouth over the muzzle thing. I don't think we can "get it" all. So, simply doing one part of the sequence of the body positioning...ie: something over the muzzle, may be misinterpreted by the dog, or may not be. So, I turn my attention more toward mamillian learning theory rather than putting a huge emphasis on dog to dog communication, although certainly there is merit in a lot of it. Anyhow, to get back to the main thing.....anything which forces the dog to "behave" or offer a behavior is taking away the opportunity for the dog to think and make the "right" choice. I try to do things which will make the dog choose and if the motivation and reward is more valuable to the dog than what it is he's doing, (the unwanted behavior) he will make that choice that we want. If he's jack potted for choosing the wanted behavior, that reinforces it so much better than when he doesn't think or choose, but is forced....not given a choice. There's only one choice when he's forced. And I don't think it sets or gels a behavior as well.
You have to make it so he WILL choose the wanted behavior though. And when this starts happening, the whole learning process, even in all kinds of other areas tends to create a more reliable and more deeply learned behavior....more solid. IMO.
Teaching attention on you, as a seperate lesson, away from these distractions helps. Desensatizing and habituation to the stimuli helps. Distraction from the stimuli and offering an alternative behavior helps.