I've wondered this myself. I traded my velvet cap and tall boots for gaited horses this past fall, training horses in preperation for trail riding. I haven't worked with any pacers yet- only fox trotters and racking horses. My general impression of gaited horses is that they are very very willing horses, and once you've built a relationship with them, they will do anything you ask of them.
I would think that pacers would be a little unbalanced for the sharp turns and such of jumping, considering that they move on the laterals. But as I said, I've never ridden a pacer, so I wouldn't know.
Rackers and fox trotters can pick up quite a bit of speed- but gaited horses were designed and bred for endurance, not power.
Keep in mind that gaited horses (non-pacers, that is) have a rapid four-beat gait, which keeps you very centered. You don't ride a gaited horse in the forward seat, which might be harder to adapt to if you're used to remaining in the forward position throughout a course. The four-beat gait might make it difficult to count off strides- before I stopped jumping, I got into the habit of whispering a little countdown to my takeoff point- if you've ever heard a horse rack down a gravel road or sound board (a panel in the floor of an arena that enables the rider to hear the horse's individual hoofbeats, to check for squareness. A very popular luxury in the gaited horse world), you'd know that the footsteps are very very close together- the horses step about two to four times faster than a trotting horse, sometimes faster- it will be hard to count. I don't know that gaited horses will have enough forward impulsion to clear a fence of much size.
However... I think it mainly depends on the horse. Ask the owner what they think the horse might be capable of.
