In order to change the duration of the dropped appointment you will need to override the ConvertDraggedData() method in the ScheduleViewDragDropBehavior.
We do not have a sample project with drag and drop functionality between GridView and ScheduleView but we have one between ListBox and ScheduleView controls. The approach used in that project is the same as the one you should use with GridView and ScheduleView.
We attached the project for you to take a look at. In it you will need to change the logic in the ConvertDraggedData() method in the ScheduleViewDragDropBehavior.cs file in order to make it work with GridView.
If you have any followup questions do not hesitate to contact us again.