To change your gantt display options, you just need to click on the Gantt Display button in the toolbar. (Not seeing it? Be sure that you've got the gantt chart open on the page.) Now, let's take a look at what you can do.
The first three options on the list - Bookings & Vacations, Phases & Milestones, and Project Date Fields - provide the ability to display the respective information on the gantt. For example, if I click on Phases & Milestones, the gantt will be populated with colored bars that represent the start and end dates for each phase in the Project Timeline app. Hovering over the bars reveals the detailed date information via a tooltip.
Show All and Hide All allow you to quickly toggle all of the above options on and off with a single click. This is particularly handy for when you are in the scheduling zone and need to flip between high-level and more detailed views.
The final menu option - Advanced - contains three additional display options. The first two let you display fields to the left and/or right of a task's gantt bar and the third lets you set the Group Bar color.