You could get the visible GroupRows using the ChildrenOfType method. An example with the first GridViewGroupRow would be:
GridViewGroupRow groupRow =
GroupViewModel groupViewModel = groupRow.GroupViewModel
header = groupViewModel.Header;
Generally I would better advise you to predefine the GroupHeaderTemplate of the GridView/GridViewColumns although I am not sure if this will meet your needs. Please check the Totals WPF Demo for an example on how to predefine it.
the Telerik team