Hello Alberto,
The ASP.Net Core Gantt is a wrapper of the jQuery Gantt. That is why, if you wish to extend or customize a functionality or layout, that should first be done in jQuery. And if you wish to configure a custom view through the HTML/Tag helpers, you have to extend their logic in the ASP.Net Core source code as well.
I hope the above explains the case. Feel free to contact us whenever you have questions.
Regards,
Martin
Progress Telerik