Top Menu is launched by swiping downwards from the the middle of the top edge of the device. Previously (in Sailfish 1 and 2) the same gesture was used for turning off an application.
An app can be closed with the gestures shown in the picture below. Always start the swipe from one of the top corners of the display. The same gestures also work when the device is in landscape position.
There is no dedicated setting for this gesture in Settings menu, i.e. it is always enabled.
Picture: Gestures to close the current application.