You have two opprotunities to achieve desired result:
1.You can use localization and the resource you have to localize is GridViewGroupPanelTopTextGrouped.
If you want to read more about Localization, please see the following article.
2.The second one is to edit the template of the GridViewGroupPanel, find the panelTextGrouped, change its text and delete the LocalizationManager.ResourceKey that is set by default.
I am attaching a sample that demonstrates sample approach.
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