A Gantt chart shows how long things take: each bar starts at a date and runs for a duration. In Tableau it needs three things:
- a continuous date on Columns, the start (for example the green DAY(Created Date));
- the items on Rows, one row per ticket, task or order;
- the Gantt Bar mark type, with the duration on Size.
On a date axis, Size is read in days, so a measure like DATEDIFF('day', start, end) gives each bar the right length. A duration on Columns instead would draw ordinary bars from zero, which is a different chart.
Watch for items that haven't finished: their end date is NULL, so the duration is NULL and the bar has no length. Decide what they should show, for example "open until today", with IFNULL.