The ability to edit inline within the Gantt view is there to optimise using the Gantt within Team meetings, or when multiple changes need to be made to tasks, from within this view.
To enable the editing inline function, simply click on the pencil icon to the right of the page. This will turn purple when the feature is enabled.
Once activated, you'll be able to edit the following inline through the left side of the Gantt
Task Start Date
Task Due Date
To achieve this, simply click on the field, and the editable state will appear, as seen below;