To access Gantt Settings go to "Actions" menu in Process Builder and select "Open Process Settings" menu item. Then navigate to Gantt Settings tab (1) of popup appeared:

On "Gantt Settings" tab user can set these settings:

• Calculate dates before starting the process (2)- if check, the Planned Record will be created as far process instance will be started. This Planned Record will have Planned Start and End dates calculated for each (active, pending and future) step. This is actual for "Date Driven" calculation modes (4) - in Duration Driven calculation mode dates are shown for estimate purposes only and are not saved.
• Calculate durations (3)- use this option if you want to make durations specified as expressions be pre-calculated and static in Gantt chart, otherwise they will be shown as formulas and calculated dynamically each time you open Gantt chart of planned or started record.
• Default calculation mode (4) - select how other steps dates will be changed if change dates/duration of some step. See proper article to know more.
• How to handle manual date changes (5) - select how steps should behave when their dates will be changed.