The Gantt Chart Summary overview is a customizable pop-up (A) that gets displayed anytime a user makes a single-click action on a project bar on the Projects Tab > View > Gantt Chart.  To customize the overview (B) go to Settings -> Account.  You have the option to select from out-of-the-box and your own custom project attributes.  Drag-and-drop the the rows to adjust the display order in the pop-up.