There are gaited mules! Even though I owned a mule ~6 or 7 years ago, I never could get past the ears and the braying. That rascal would constantly find her way out of my pasture and take the horses with her. Once I sold her, the escaping stopped.
Just like any breed, there are good and bad in gaited horses. Once you get used to the smooth ride, many riders never go back to QHs again.