Gas line for direct vent fireplace
I'm putting in a direct vent fireplace in an existing fireplace alcove. The existing gas line ends with a gas ball valve and 1/2" NPT female. The gas line is black pipe to the meter. The fireplace, Medota Greenbriar, has a 1/2" female NPT fitting. The space where the gas line connections will be enclosed behind wall board, so I can't use a union.
Question: Are the flexible lines used to connect appliances suitable for use behind enclosed walls? Are there flexible lines available use use behind enclosed walls?