I'm not a fan of the Imus saddles. The quality I've observed is spotty. They are made by subcontractors. Not all subcontractors create equally. They are quite pricy.
The tree in a saddle has multiple jobs, but one critical job is weight distribution. The "treeless" saddles generally do not do this job well and you end up with two, big pressure points under the rider's buttocks. This is, in general, a Very Bad Thing.
The "gaited horse saddle" is more a marketing angle than a design feature. Every saddle needs to fit such that the horse has adequate shoulder room. Horses jumping 60" in a puissance or running a cross country course need at least as much room as a Walker gaiting around a flat track!
Saddles made of cheap materials are almost always of cheap design, too. Remember when you examine a saddle that if you're not impressed by what you see then you should be very concerned about what you can't see (like the tree).
After you strip away the "night soil, bovine, male" from the saddle business you're left with some basic principles of fit.
The easiest way to fit a horse is to load it up and head for the biggest tack store you can find. When you get there start trying them out 'till you find one that works for you and the horse.