Its the same concept, however, ones at a gym are not going to be as accurate as a SRM or Powertap. Just changing your position from the hoods to the drops will change your power output so someone that can only put out XXX on a gym machine doesn't mean that he won't be able to put out more or less on a road bike. There are some gyms that have high end exersise bikes like the ones made by the makers of Powertaps, but at least at the gyms near me, a tricycle is a better representation of a real bike.