Mike, there is approved flexible gas piping available however it must be installed by a liscensed plumber who has passed courses in it's correct installation. While it does fit your criteria, it is a relatively new product and I would not use it until there is more experience in service. Also, I would not have it installed in places where it was possible for someone to grab on to it and pull in any direction.
Also, ASTM A-53 black pipe has been the standard for this service for many years with a great track record. There is a lot of this product coming from offshore and the quality of it does not match that of the domestic product. The pipe that you buy in home centers is usually import. My strong preference is steel pipe from a domestic manufacturer and this is available from plumbing supply houses. It is available in 10 and 21 foot lengths and, normally, cut and threaded to your specific required length.