At this stage there is no built-in way to achieve this, but you could easily override the GanttItemsPresenter Style and redefine the MinWidth of each column in the RadGanntView. The columns in the GanttItemsPresenter template are defined like following:
<telerik:GanttPresenterPanel.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" MinWidth="3" />
<ColumnDefinition Width="3*" />
<ColumnDefinition Width="Auto" />
</telerik:GanttPresenterPanel.ColumnDefinitions>
The first column is for the frozen columns, second for the rest of the GanttView grid, the third for the timeline panel and the fourth for the ScrollBar at the end. I've also attached a sample project which demonstrate this solution.