How to track a workout with the Apple Watch app?
First, make sure SmartGym has the right permissions to read your Health data. If you're unsure about this, you can check this helper article.
After the Health app is properly configured, tap Start Workout on the main screen of SmartGym on your Apple Watch. This will start tracking all your health data like calories, heart rate and distance.
Then, just choose the routine you want to do and you're good to go.