By default the Gantt chart shows the time range as small as to fit the items.

If you want to set the start and end date manually, for example to allow the dragging of the items to future, set following widget property in Advanced > Gantt Config Script (both need to be set).

gantt.config.start_date = new Date(2018, 12, 10);
gantt.config.end_date = new Date(2020, 08, 20);


Please, update to 1.6.1 before using next solution.

What if you want to connect it to Page Parameters of your LiveReport page?

Let's suppose you have two parameters, start and end:

gantt.config.start_date = new Date($widgetContext.pageParameters.start.value.time);
gantt.config.end_date = new Date($widgetContext.pageParameters.end.value.time);