I am afraid that the desired functionality is not achievable. This is due to the fact that the cells in the grid portion of the RadGanttView need to be aligned with the rows in the timeline portion of the control, which is by design.
That said, as an alternative approach I can suggest distinguishing the tasks in some other way. For example, by bolding their titles. You can use a custom CellTemplate in order to change the appearance of specific tasks.
I hope you find this helpful.
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