RibbonGroupBox inherits from HeaderedItemsControl. That way you can use things like Header, HeaderTemplate and so on.
Because the header has to look different when the control has it’s State set to Collapsed RibbonGroupBox offers the attached property IsCollapsedHeaderContentPresenter on the ContentControl and ContentPresenter being visible in that state.
The style for the ContentControl being used in that case is Fluent.Ribbon.Styles.RibbonGroupBox.TwoLineHeaderContentControl.
If you don’t want to show different templates you can simply overwrite the set HeaderTemplateSelector.
If you want to show different templates, but don’t want to create your own HeaderTemplateSelector you can provide DataTemplate resources with keys as Fluent.Ribbon.Templates.RibbonGroupBox.OneLineHeaderTemplate and Fluent.Ribbon.Templates.RibbonGroupBox.TwoLineHeaderTemplate.