Two ladies on my local horse forum use treeless saddles on their Tennessee Walkers.
One uses a Bob Marshall and says there's nothing better.
The other uses a Hillason (sp?) and says there's nothing better
It seems, overall, that more folks are happy with the Bob Marshall than the Hillason.
I have Tennessee Walkers but I still ride bareback all the time, so I am of no help.
If any of my long-time riding friends do come down and ride, I have a custom-made, partly synthetic Crestridge that I bought used (Crestridge saddles are specifically for gaited horses), and I have an off-brand 11 pound endurance saddle that I use on the other Walker.
Neither saddle is a perfect fit for either horse, but they both work well for my very experienced friends to ride 3-4 hours of hard trails.
Neither horse ever came back galded or sore.
Even when I used to slide horses on their butts down the hill side, I never used a saddle. If I were still riding that hard and did need a saddle, I would probably order a custom-made Crestridge of the lightest weight, which would include being synthetic.
There is also a gaited saddle maker called "Allegheny Saddles" or "Saddlery". I can't remember. You should be able to Google either one of these saddle companies.
The Crestridge and the Allegheny saddles are NOT treeless.
I would be happy to find the links, but I am running a little behind and need to get to the barn, then take our generator to the repair shop thanks to the chipmunks or mice
Hope this helps you a little bit