Apple Watch can pair and sync with compatible gym equipment, by following the instructions here:
It may otherwise help to calibrate your Apple Watch, including resetting any existing calibration data.
Calibration teaches your Apple Watch how your arm movements relate to your stride lengths at different speeds when you are walking and/or running. It does this, during outdoor walk and/or outdoor run workouts that are recorded via the Workout app, by comparing data from the accelerometer with GPS (Location Services) data.
To calibrate your Apple Watch - and to reset existing calibration data - follow the instructions here:
Resetting your calibration data can be equally as helpful for Series 2 & 3 models and will not erase your Activity history.
For the best results, be sure to allow the arm on which you are wearing your Apple Watch to swing naturally during treadmill-based workouts and also during calibration.
Keep in mind that, when estimating distances for indoor walks and runs via the built-in accelerometer, Apple Watch cannot recognise or adjust for inclines or declines on a treadmill.