The drag and drop is completely encapsulated within FlexGanttFX, so you can not work with the DnD API. What you need to use are the callbacks on GraphicsBase. In this case the “row drag and drop callback”.
* Specifies a callback that will be invoked when the user drags an activity
* over a row of the given type. The callback implementation then determines
* if a drop would be accepted in the given row.
public final void setRowDragAndDropCallback(Class< ? extends Row> rowType, Callback callback);