To reproduce: 
1. Drag and drop RadGanttView
2. Add summary task with 3 sub tasks
3. Set the ReadOnly property to true
4. Run the form and try to move summary task. The user should not be able to move summary task or any other task when is in read only mode. 

Workaround: 
You can set the following properties: 
this.radGanttView1.ReadOnly = true;
this.radGanttView1.AllowSummaryEditing = false;