Is a Java applet an option for you? E.g. http://www.bdaum.de/howto.htm describes how to use one. See http://www.coderanch.com/t/259381/Applets/java/display-CGM-images-iingn-web for more discussion.
As an alternative, you could convert your CGM files to SVG or PDF on the server. You could do that either in advance, or on-the-fly, depending on your requirements. E.g. http://www.cgmlarson.com/convert_CGM_to_PDF_aerospace.html has a server license. (I don't know anything about the suitability or price of the product.)