The issue you are experiencing is identified on our side and is related to the integration of our controls with Blend 2015. We have it logged in our internal system. You can follow the item at the following page from our Ideas & Feedback portal - Missing Toolbox Items in Blend 2015. This way you will receive a notification once a major event has been registered.
As a workaround, you can use Blend 2012, or extract the GridViewCellTemplate from the Telerik.Windows.Controls.GridView. file. I have added a sample project that implements the latter. Please review it and consider such approach at your end.
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer and share your thoughts.