The aim is to end the sleep mode and open a specific app through a defined gesture.

Go to the All Apps section  and open Settings then OffScreen Gesture.

To link an app to a specific gesture on the screen with the device in sleep mode:

1. Turn on the Offscreen gesture option.
2. Select the gesture to be used.
3. Select the app to be linked with the gesture.

 Note that doing a double click allows to wake up the phone without opening any specific app. Some gestures are predefined and can't be modified. Moreover, you can't create new gestures.