CGI files from Web Servers
The CGI extension does not tell you much about the contents of the file, but rather that it has been generated by a web server. Usually it will be a HTML document which can be opened by all text editors, but it may be other file types as well.
If the CGI file is HTML you can save it as a Web Archive with Safari and drop the archive on File Juicer to extract what is inside the archive.
You can drop CGI files on File Jucier to see if it contains any of the 20 formats File Jucier can extract.