In order to achieve your goal, you need to manipulate the Disabled state of GridViewCell element. Please check the Styling a Cell help article for more information. I have prepared a sample project using our Implicit Styles. Please check the changes I made in the Themes/ Telerik.Windows.Controls.GridView.xaml file.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.