Ok they're my favourite breed of horse and the only type I've ever owned or ridden for more than 20min in total.
The gait is rack or well call it tÃ¶lt but I was going to give points for rack as well. It's absolutely fantastic to sit.
Here's a really good clip that shows the gait in motion:http://www.youtube.com/watch?v=-7rWeWymJDw
You can hear a perfect four beat rythm and you can see the rider barely moves. They sometimes hold a beer glass full to the brim in one hand when riding particularly smooth horses in shows and it doesn't spill. It's really fun and comfortable to ride. A lot of horses don't go quite that fast though and don't lift the legs up that high. Mine didn't but he was great though.