How can a user download a file in client side (Google Web Toolkit)

You REALLY need to distinguish between GWT client side java code and server side java code.

On the client side in your GWT Java Code

String url = GWT.getModuleBaseURL() + "downloadService?fileInfo1=" + fileInfo1;
Window.open( url, "_blank", "status=0,toolbar=0,menubar=0,location=0");

On server side in your non-gwt Java code-

In web.xml

<servlet>
    <servlet-name>downloadService</servlet-name>
    <servlet-class>AAA.BBB.CCC.DownloadServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>downloadService</servlet-name>
    <url-pattern>/<gwtmodulename>/downloadService</url-pattern>
</servlet-mapping>

In server package code a servlet

    public class DownloadServlet extends HttpServlet{
    protected void doGet( HttpServletRequest req, HttpServletResponse resp ) throws ServletException, IOException
        {
            String fileName = req.getParameter( "fileInfo1" );

            int BUFFER = 1024 * 100;
            resp.setContentType( "application/octet-stream" );
            resp.setHeader( "Content-Disposition:", "attachment;filename=" + "\"" + fileName + "\"" );
            ServletOutputStream outputStream = resp.getOutputStream();
            resp.setContentLength( Long.valueOf( getfile(fileName).length() ).intValue() );
            resp.setBufferSize( BUFFER );
            //Your IO code goes here to create a file and set to outputStream//

        }
    }

Ensure you push your file contents to **outputStream** .

If you know the path of the file, Code snippet is shown below.

button.addClickHandler(new ClickHandler()  
{ 

    @Overrid
    public void onClick(ClickEvent event) 
    {
        Window.open(GWT.getHostPageBaseURL() + "/file.rar", "name", "enabled");
    }
});