Hi Gilbert van Veen,
Generally there are two ways to achieve this result:
1. Create a new template for the GridView's GroupPanel in Expression Blend.
2. Change the localization strings for the GridView for the language you are using.
Please let me know if you need any further assistance.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items