In teh current version of RadGanttView there isn't a built-in feature that could be used in order to directly disable the TabStop of the control's GridView part. In order to change the IsTabStop of that part you will need to customize all of the Container Styles of the grid part and set their IsTabStop to false.
I created and attached a sample project with illustrates the describe appraoch, hope this is helpful.
Regards,
Vladi
Telerik
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.