Indeed, you have use the exact same keys - in this case GridViewGroupPanelText instead of GroupPanelText. Otherwise the grid will not be able to associate the localized string to the corresponding key. You may take a look at our online documentation for a reference on the keys used in the RadGridView.
Furthermore, you may define all the keys and their corresponding values in a single file. There is no need to add separate files for each control.
All the best,
the Telerik team