In fact, Gerber is an international standard lithography format file, including RS-274-D and RS-274-X formats.
RS-274-D is the basic Gerber format, which requires a D code file to fully describe one drawing;
While RS-274-X is an extended Gerber format so that it contains D code information.
BTW, all commonly used CAD software that are able to generate the above two format files.
