foreach (GridViewRow gvr in GridView1.Rows)   //loop through GridView
        {
            OleDbConnection conn = new OleDbConnection("your_connectionstring");
            //suppose ItemCode is in the first column and Quantity is in the third column of GridView
            OleDbCommand cmd = new OleDbCommand("insert into [QDETAILS](ItemCode,Quantity) values (" + gvr.Cells[0].Text + ",'" + gvr.Cells[2].Text + "')", conn);
            conn.Open();
            cmd.ExecuteNonQuery();
            conn.Close();
        }