Thanks for contacting Syncfusion support.
We can add, remove and insert the items from the item source collection of the Gantt Control as in the below code example
(gantt.ItemsSource as ObservableCollection<TaskDetails>).Add(new TaskDetails
StartDate = (gantt.ItemsSource as ObservableCollection<TaskDetails>)[count-1].FinishDate,
Duration = new TimeSpan(2, 0, 0, 0)
Also, we have modified the sample for your reference. You can download the sample from the following location.
Note: InbuildTaskCollection property is provided to access the task items when item source is imported from the xml which makes it easier to access the task items.