1. Add the grid into a div control:

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HtmlGenericControl div = new HtmlGenericControl();
div.ID = "GridContainer";
div.Controls.Add(this.GridControl);

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2. Use javascript to open, send to print and close the Grid in a new window:

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private void Print()

{

            _gridControl.DisplayLayout.ReadOnly = ReadOnly.PrintingFriendly;

            string script =
                        "<script language=javascript> " +
                        " var prtContent = document.getElementById('" + div.ClientID + "'); " +
                        " var WinPrint = window.open(); " +//'','','letf=0,top=0,width=1,height=1,toolbar=0,scrollbars=0,status=0'); " +
                        " WinPrint.document.write(prtContent.innerHTML); " +
                        " WinPrint.document.close(); " +
                        " WinPrint.print(); " +                        
                        " WinPrint.close(); " +
                        "</script> ";

            Page.RegisterStartupScript("Script", script);

}

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I found this useful.

Using custom rendering through HtmlTextWriter may indeed interfere with how the grid renders and prints. I have just read what Roger suggested and I believe his ideas are spot on - PDF is specifically used for printing, end-users are used to it and you have greater control there, so maybe you can implement print preview option in your site that essentially exports to PDF and end-users can print from there.

@Roger - thanks a lot for sharing this approach in forums. We will be working hard on updating the document export documentation for PDF/Excel/Word.