I misunderstood meaning of flick gesture so I missed that it was a reported bug although I did a search(for mapgesture).
https://bugreports.qt.io/browse/QTBUG-46388
It is fixed in 5.6 but not 5.5.
However there is a simple workaround. You have to add a MouseArea to the Map element as shown below :

Map { id : map plugin: myPlugin anchors.fill: parent focus: true zoomLevel: 9 gesture.activeGestures: MapGestureArea.PanGesture | MapGestureArea.FlickGesture | MapGestureArea.ZoomGesture gesture.flickDeceleration: 3000 gesture.enabled: true //XXXXXXX Added MouseArea { anchors.fill: parent } //XXXXXXX }

Thanks to Alexander for pointing me this.