Esta es una manera fácil de enviar desde GWT archivos al navegador para que los usuarios se los puedan descargar.

1. Hay que agregar un frame oculto en el HTML de la hosted page:

<div id="__gwt_downloadFrame" tabIndex='-1'></div>

2. Para iniciar la descarga poner este código en la parte cliente (código GWT):

public static void download(String p_uuid, String p_filename) { 
        String fileDownloadURL = "/fileDownloadServlet" 
                               + "?id=" + p_uuid 
                               + "&filename=" + 
URL.encode(p_filename); 
        Frame fileDownloadFrame = new Frame(fileDownloadURL); 
        fileDownloadFrame.setSize("0px", "0px"); 
        fileDownloadFrame.setVisible(false); 
        RootPanel panel = RootPanel.get("__gwt_downloadFrame"); 
        while (panel.getWidgetCount() > 0) 
            panel.remove(0); 
        panel.add(fileDownloadFrame); 
} 

3. Poner esto en el servlet que va a servir el archivo a descargar:

@Override 
protected void doGet(HttpServletRequest p_request, 
                         HttpServletResponse p_response) 
        throws ServletException, IOException { 
        String filename = p_request.getParameter("filename"); 
        if (filename == null) 
        { 
            p_response.sendError(SC_BAD_REQUEST, "Missing filename"); 
            return; 
        } 

        File file = /* however you choose to go about resolving 
filename */ 

        long length = file.length(); 
        FileInputStream fis = new FileInputStream(file); 
        p_response.addHeader("Content-Disposition", 
                             "attachment; filename="" + filename + 
"""); 
        p_response.setContentType("application/octet-stream"); 
        if (length > 0 && length <= Integer.MAX_VALUE); 
            p_response.setContentLength((int)length); 
        ServletOutputStream out = p_response.getOutputStream(); 
        p_response.setBufferSize(32768); 
        int bufSize = p_response.getBufferSize(); 
        byte[] buffer = new byte[bufSize]; 
        BufferedInputStream bis = new BufferedInputStream(fis, 
bufSize); 
        int bytes; 
        while ((bytes = bis.read(buffer, 0, bufSize)) >= 0) 
            out.write(buffer, 0, bytes); 
        bis.close(); 
        fis.close(); 
        out.flush(); 
        out.close(); 
}