First, you will need to implement custom DragDropBehavior and ResizeBehavior of the GanttView in order to prevent dragging and resizing of tasks. The approach is demonstrated in the Custom Drag and Drop Behaviors
As to the Grid part - you could set IsReadOnly property of the controls inside the CellEditTemplate in case at some point editing will be allowed, or completely remove the CellEditTemplate.
I hope this would be helpful.
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