gridView.OptionsView.ShowFooter = true;
gridView.Columns["Mark"].Group();

// Creating total and group summaries
GridColumn column = gridView.Columns["Length"];
column.SummaryItem.SummaryType = SummaryItemType.Sum;
// format the total summary
column.SummaryItem.DisplayFormat = "Total: {0:n3}";
// group summary
gridView.GroupSummary.Add(SummaryItemType.Count, "Mark");

gridView.GroupSummary.Add(new GridGroupSummaryItem() {
  FieldName = "Length",
    SummaryType = SummaryItemType.Sum,
    DisplayFormat = "Sum: {0:n5}",
    ShowInGroupColumnFooter = gridView.Columns["Length"]
});