The app is actually capable of differentiating between four different movements, one for each axis of rotation in the three spatial planes and another that consists in shaking it horizontally. Each one of them can be associated to an independent action, like anything from opening an app to launching a specific service or even activating a special characteristic on your smartphone like the flashlight function or your WiFi network manager.
You do not need to open this application manually. Any time your smartphone screen is lit, Gravity Gestures will automatically start up. All you have to do is activate it once from your settings menu so that it stays on and you can activate its response by gesturing on your screen whenever you want.