The element inside the GridViewGroupPanelItem is actually an Image taken of the contents of the Header cell. This is done as a performance optimization for scenarios with custom elements such as the one in the ColumnSelection SDK example.
With this in mind, there isn't an out of the box way to achieve the desired requirement. That said, you can inherit the GridViewGroupPanel class and override its PrepareContainerForItemOverride method. You can set the Content after the base method has set an Image in order to override this behavior. Of course, you would also have to replace the default GridViewGroupPanel with the custom one, by modifying the ControlTemplate of the RadGridView.
I am attaching a sample project demonstrating this approach for your reference. Please, check it out and let me know if it helps. Note, that the recommended way of modifying ControlTemplates is with implicit styling and the NoXaml binaries as demonstrated in the project. Additionally, note that the extracted ControlTemplate is for the OfficeBlack theme. If you are using a different one, you will have to extract the corresponding template for that theme.
I hope you find this helpful.
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