QH equals quarter horse. Most of your western saddles are made for quarter horses. They have a rounder back with low withers kinda shaped like a barrel. They have a frame that is basically straight bars that are longer front to back. This type of saddle put on a walker, will put weight on his withers, and the back will dig into his hip right where the spine goes into hip. THink of someone putting a knuckle right on your spine.
A gaited saddle as opposed to a QH saddle bars flare at the front to give clearance for the shoulders and it has a higher arch under the horn to clear the withers.
Lots of reading here: http://www.horsesaddleshop.com/saddlefit.html
SOunds like you should be looking for a saddle specifically labled as "gaited"