Perhaps a working sample of the Gantt Chart widget would help.
For this widget: https://marketplace.mendix.com/link/component/2969
The source code is here: https://github.com/Criccle/GoogleGantt
And there’s a Mendix test project here: https://github.com/Criccle/GoogleGantt/tree/master/test
Clone that repository and open the project in Studio Pro to see how the widget is used.
Thanks a lot Eric, that helps us to proceed, thanks