I disagree on any "absolute" number for saddle fit on any class of horse. Withing a given type of horse you can have a very narrow range of size. When the Army ran the Remount Service they produced a TB style horse that was so consistent in size that the Phillips Officers Saddle, the last official saddle of the U.S. Army, it was made in only one tree size.
So one size can fit all if all are the same size.
With gaited horses since you can run from a Paso Fino to an Icy to a Walker to a Marchador to a Peruvian to all manner of crosses the idea of one "gaited saddle" is just silly. Even within breeds such is very difficult unless the breed is so uniform that it approaches Remount Service consistency.
Well engineered and built saddles will accommodate a range of horses. There is no real necessity for an individual saddle for each horse. But the "gaited horse saddle" remains a marketing approach, not an operational type.