DataGridView Join

You should be querying the datatable, not the dgv.

           DataTable resultTbl = new DataTable();
            resultTbl.Columns.Add("Item No", typeof(int));
            resultTbl.Columns.Add("Qty", typeof(decimal));

            object[] result = (from row in getgrnstable.AsEnumerable()
                         group row by new { rawitemno = row.Field<string>("rawitemno") } into grp
                         select new 
                         {
                             rawitemno = grp.Key.rawitemno,
                             linkedqty = grp.Sum(p => p.Field<decimal>("linkedqty"))
                         }).ToArray();
            foreach(object[] row in result)
            {
                resultTbl.Rows.Add(row);
            }