There are two scenarios to keep in mind.
1) You cannot drag the gantt on the Portfolio default plan.
You will need to create a new plan and switch to that plan.
Then you will be able to drag the gantt.
2) For dragging the gantt, you need to select the whole gantt, not by clicking on the end where your mouse then shows arrows and trying to extend it.
Select the whole gantt and then you can drag it.
Note: The duration remains the same, therefore the gantt bar stays the same size.
The duration works the same if you try changing the Start or Finish date on the Waterlines/Investment list as well. When one date changes the other date changes to keep the duration.