How to print GridView in LandScape Orientation in ASP.Net

I am using this code for printing ,. i want to set print as landscape ,.

 GridView1.UseAccessibleHeader = true;
        GridView1.HeaderRow.TableSection = TableRowSection.TableHeader;
        GridView1.FooterRow.TableSection = TableRowSection.TableFooter;
        GridView1.Attributes["style"] = "border-collapse:separate";
        foreach (GridViewRow row in GridView1.Rows)
        {
            if (row.RowIndex % 10 == 0 && row.RowIndex != 0)
            {
                row.Attributes["style"] = "page-break-after:always;";
            }
        }
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        GridView1.RenderControl(hw);
        string gridHTML = sw.ToString().Replace("\"", "'").Replace(System.Environment.NewLine, "");
        StringBuilder sb = new StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.onload = new function(){");
        sb.Append("var printWin = window.open('', '', 'left=0");
        sb.Append(",top=300,width=1000,height=600,status=0');");
        sb.Append("printWin.document.write(\"");
        string style = "<style type = 'text/css'>thead {display:table-header-group;} tfoot{display:table-footer-group;}</style>";
        sb.Append(style + gridHTML);
        sb.Append("\");");
        
        sb.Append("printWin.document.close();");
        sb.Append("printWin.focus();");
        sb.Append("printWin.print();");
        sb.Append("printWin.close();");
        sb.Append("};");
        sb.Append("</script>");
        ClientScript.RegisterStartupScript(this.GetType(), "GridPrint", sb.ToString());
        GridView1.DataBind();