No you couldn't really put a GP dressage horse in a bosal.
There is no 'setting' of anything, body or head, in dressage. There is no 'head set' in dressage, there is no 'frame' at all, there is no 'training level frame', 'first level frame'...none. It's the most misunderstood thing in dressage, and the one thing that keeps so many people from doing well or progressing in dressage.
There is nothing about gaiting in and of itself, physically, technically, that requires a curb bit or for the horse to have a specific head and neck position. Except that horse people are bull headed as all get out and whatever way they been doing something is the right way and the only way, LOL!
There are STYLES of gaiting that people want to ADHERE to, out of preference, but that has nothing to do with what has to be there, technically, physically, physics-of-motion-wise, to gait.
I've been told all of it. A horse has to have his nose poked out to gait, he has to have his neck up to gait. A horse has to be in a curb bit to gait. A horse has to be in a WESTERN saddle to gait, a horse has to be in an ENGLISH saddle.
All it is, is a horse show class, a show style a person wants to stick to, and that has nothing to do with physics or necessity.
Hundreds of years ago, MOST people rode gaited horses. AND THEY RODE THEM IN SNAFFLES. That was the 'every man' type of riding. That's what people did.
Horses can and do gait very well on a snaffle. I know because I've seen me do it. I even had an Icelandic breeder yell out 'no, no noooo!!!!' and then very quietly say, 'I suppose you'll be very happy to know that he looks just like a &%)(*#)$#*# dressage horse, now!'
The fact is, a gaited horse can gait however he's TRAINED to gait. And he also can be RETRAINED. There are a whole lot of different ways to skin a cat.
The American Saddlebred isn't even called a 'natural gaited horse' and he is trained to gait in an entirely different posture than a Tennessee Walker. In fact, every gaited show style is a little different. Then there's this - snaffle gaited horses - FIVE gaited, in fact, the best of them:
Fact is, there are books published in the late 1200's in Europe about HOW to make a horse 'thorough paced' (5 gaited) in a snaffle bridle OR a curb. It's not even new shtuff.