GridView.SelectedRow Property returns a (single) reference to a GridViewRow object that represents the selected row. That's why it's throwing error when you want apply a loop on it.
Try-
lbllist.Items.Add(gvPkgcbndate.SelectedRow.Cells[0].Text.ToString() + " " + gvPkgcbndate.SelectedRow.Cells[1].Text.ToString());
Hope, it helps :)