To produce a Gantt chart, dates must be set to each phase of the job first. You can learn how to set dates to a job here by clicking here.
To create a gantt chart for a project, first go to the Plan & Track section. At the top right corner of the list of phases, there will be a drop-down menu with a View Gantt Chart button:
It will use the dates set to each phase of the project to develop a Gantt chart, downloadable as PDF or PNG: