Some properties of the Gantt UI element are not set using the UI element properties but instead using the method handler IF_WD_GANTT_METHODS. You can access the method handler in WDDOMODIFYVIEW.
Declare a reference to IF_WD_GANTT_METHODS in the Attribute section of your view.
Assign the reference in WDDOMODIFYVIEW.
Call the method.
You can find an example in the system in component WDR_TEST_EVENTS in view GANTT.