The only sort functionality is by using issues fields. However, when using gantt, sorting by the configure start date field, it doesn't provide always good sorting when some issues have a start date and other issues are being scheduled automatically and would result in an earlier schedule start date than the manual entered start date.
Hence,
- sorting can be done on issues with a manual start date
- sorting is automatically done for issues with a auto-scheduled start date

But: sorting cannot be done between manual & auto-scheduled issues