I have managed to solve this problem by creating custom component that extends GanttView then edited the viewStartOf function as below

viewStartOf(date: GanttDate) {
    return date.startOfWeek({ weekStartsOn: 6 });
  }

You can also find more details in their github example

https://github.com/worktile/ngx-gantt/tree/master/example/src/app/gantt-custom-view