I went to a normal gaited dressage event recently, and got talking to some women about it. I told them that I am interested, but ride a walker. Two women had experience riding their gaited horses in western dressage events. I don't know if you have found western events that are FOR gaited horses, or just western dressage events in general. These women both told me that most of the judges only had experience with Quarter Horses and the likes, and some would mark them down. They said that a knowledgeable judge will compensate for the gaited horse, but some will not, and that if you aren't going for scores but are doing it for suppleness, respect, relaxation, ect., that it is totally worth it :)
I also plan on doing Western dressage on my gaited horse. Good luck!
"Putting pads on a gaited horse is like putting a mask on a beautiful woman."