Please check this : viewtopic.php?f=52&t=21871&p=108104&hilit=start+monday#p108104
Best of luck,
By default this is defined by the loaded locale. So if you need this to be changed everywhere you'd better make your custom locale.
And if you need this on the Gantt level only use this config: https://www.bryntum.com/docs/gantt/api/Gantt/view/Gantt#config-weekStartDay