How do I control the music playback from the Watch?

SmartGym on Apple Watch allows you to easily control music playback from the Watch app. Just swipe from the right to the left on any screen* to access the music controls.

*On watchOS 6 there is a bug that prevents the swipe gesture from working on the exercise's screen (any other screen will work just fine). We already reported this to Apple. Let's hope they fix it soon.