My horse was really hard to fit. It was more a matter of finding a saddle short enough (front to back) to fit his really short back, not interfere with shoulder and hip movement for clean gaiting, yet still sit right on his mutton withers.
Because I ride english and Aussie, it was a little easier to find the right length, and he does fine in a wide/full QH bars.
I learned how to take all the appropriate measurements, then called the saddle company I wanted to buy from and asked a zillion questions.
As for western saddles, I have been looking for an extra one since my friends ride western and I know from my research and conversations with the saddle fitters that he doesn't need a gaited tree saddle. A small rounded skirt or endurance type western saddle with full QH bars will work just fine for my gaited horse.
I'm not for or against a gaited tree, I just don't think a gaited horse must have a gaited tree.... just because.
Its all an individual horse thing and what works best for his/her body.