Tree-less is a great option - BUT; yu need a good treeless. Chose one with panels under it that doesn't require a special pad. My expeience is that they are better. They do need to fit, not all models fit all horses even if it's a treeless, at least not if it's a good treeless.
Cheap tree less saddles can be downright dangerous, not taying in place is dangerous to you (ask my shattered wrist) and not enough to even out the preassure from the stirrups.
Crow's ridden in almost only a treeless (star trekk espaniola) and he has developed good muscles and is constantly improving his gait as well :)
If your horse won't gait with a saddle, can it just be that he/she isn' used to gaiting with one? I know I had problems to get Crow to canter when I went from saddle to bareback, but then when I went back to saddle after learning bareback canter, we had to start over again.. xD Now he'll canter either way :)