In the current version of RadGanttView control in order for the GridView and the TimeLine area to be sized with 50% by 50% you will need to customize the GanttItemsPresenterStyle
's Template. In that Template all you need to do is set the second and third ColumnDefinition Width's to 50*. You should not that if there are frozen RadGanttView.Columns in the current version of the GanttView control it is not possible to achieve a 50 by 50 sized areas.
Another approach for achieving the desired behavior is to completely replace the GanttPresenterPanel in that Template with a normal Grid and set its ColumnDefinitions/RowDefinitions. It is worth mentioning that if the control has big data changing the GanttPresenterPanel with a normal Grid could break the built-in virtualization of the control and there could be some performance issues.
We created and attached a sample project for you with the described approach, hope this is helpful.