Hi Michal,
Thanks for contacting Syncfusion support.
We have analyzed your requirement. Currently in Gantt there is no support to render the tasks without startDate and endDates. But we have already logged the feature report as “Support for unscheduled tasks in Gantt control” and this feature will be available in our upcoming 2018 Volume 3 main release, which is expected to be roll out in the end of September 2018.
Please let us know if you have any concern.
Regards,
John R