yup, 26" trials bikes (or certainly used to) have gears, a minimum of 6 working gears for competitions, thats why the hope ProII etc are all available in a "trials" version with a small freehub, the idea being to butcher a 9s cassette by removing the unwanted gears, getting a stronger wheel and saving weight.
20" trials bikes dont as they derived from BMX's and so retained their singlespeedness.
IIRC the rules were relaxed a few years back though to allow stuff like 22 & 24" wheels, suspension forks etc, and I think all bikes are now able to compete against each other (rather than the different comps for differnt bikes)?