A horse's gait is first determined by it's DNA. The conformation and "brain wiring" will determine the sequence of movement.
Then the human enters the picture and can alter this "base" gait by husbandry practices (like shoeing, trimming, etc.). The human can apply devices (pads, chains, etc.). The human can use tack to alter gait (saddle positioning, bits, etc.). And the human can use riding position to alter gait (moderating pressure on the back with the seat, use of hands, etc.).
Finally the horse's fitness and strength level will determine the quality of movement.
Different gaited breeds have been selected for different movements. The Walker and Paso Fino are both gaited, but have a rather different movement.
It is well to keep in mind that there are few, if any, universal
principles that apply to "gaited horses."
A horseman knows that if you continuously repeat any action beyond the horse's ability to perform that action then you're asking for trouble. In the case of the back if you continuously hold a "ventroflexed" position and don't allow the horse to "flex" by rounding from time to time that a sore back is as sure as night follows day. This is independent of breed. So a horseman mixes their gaits as they work.
For the show horse this is not such a big deal as the time in the ring is minuscule compared to the training time at home. So even extreme ventroflexion, in a properly prepared and conditioned horse, will not cause long term difficulty by itself. Move the horse to the trail and the problem becomes more complex. Here, even a minor ventroflexion will cause soreness if not relieved by gait alteration because the horse will be working for hours, not minutes. Again, this is independent of breed.
Head set can be had two ways. The classical head set is obtained by the rider using the leg to engage the hind end and "capture" it's power, then use the hand to "meter" the power to the front end. This will result in the horse balancing
itself and putting its head where it can best use that power for horizontal motion (a rider can "tweek" this a bit, but not by much, or it will upset the balance). Or the rider can use the hand, in conjunction with the bit (or some other device), to "set" the head in some arbitrary position. This frequently results in back problems because the rider has disconnected the front end from the back end and sacrificed the power of the back end. This is true whether you're talking about a "peanut rolling" QH, a rollkured warmblood, or "star gazing" gaited horse.
The gaited horse world has a very bad habit of obsessing about gait, to the exclusion of the rest of the horse. While I hate to use the word "holistic" (because the Fruitcake Community has so badly abused it) a horseman always rides the whole horse. While they might have to "zero in" on some aspect to address some problem they must "put the horse back together again" before they step into the leathers.
The rules of equine biomechanics apply to gaited horses and trotting horses alike. There are some differences, but they are mostly subtle. I flatly reject the notion that "all gaited horses must have a high head set" or that "all gaited horses must be ridden in a curb bit" or any other such "universal rules." Proponents of such rules are demonstrating an ignorance of horsemanship. There are general rules within a breed (we call them "breed standards"). These "standards," however, are humanly generated and reflect a desire for certain traits. They are certainly not Rules of the Universe.
Back health is critical in any horse if they are to be able to do their job. But so is foot health, gut health, leg health, mouth health, etc. A horseman knows to be wary of arguments over which body part is most important (and for those who remember that old joke the punch line still stands