To answer your question we must review the history of Black iron versus galvanized iron pipe. In years past black iron pipe was made as a seamless pipe. A raw donut of iron was super heated then physically stretched to about 30 feet to form a tube. The seamless tube was then reheated and passed through forming dies to reduce it to its final diameter.
Galvanized pipe was made in a continuous forming operation where a flat piece of steel was heated and formed though dies until it was rounded to the final pipe shape and continuously welded for its entire lenght. After the raw pipe was formed it then was passed through the zinc plating process, cut into finished lengths and pressure tested prior to passing final inspection. The concern was that if the zinc coating were to corrode away it would then leave the weld exposed to corrosion and could introduce a leak in the pipe. For that reasons only seamless black iron pipe was approved for gas.
The codes now approve Black Iron Pipe, galvanized pipe, continuous drawn copper pipe (Copper pipe may only be used if the gas is certified to contain less than 0.3grains of hydrogen sulfide per 100 cubic feet-requires a certification letter from the gas supplier- and the copper pipe MUST BE labelled as approved for gas.)
Polybutelene and polyethelene is approved for direct burial underground service only. (When plastic lines are run below grade you must also install an AWG18 yellow insulated copper trace wire parallel to the line to permit tracing the line with a tone generator.)
The International Residential Code also permits CSST (continuous stainless steel Tubing) but it is prohibited by the Uniform Plumbing Code.
While these materials may be listed as approved by the major codes, many local codes still limit gas inside a structure to black iron pipe.
It must also be noted that in those areas where other materials may be approved, you MAY NOT mix materials on a run. If its black pipe, all fittings must also be black pipe.
Copper may not be directly coupled to iron pipe, but rather you must use a dielectric union approved for gas or a 6" hardened bronze nipple to make the transistion. Copper pipe requires brazed joints or approved threaded fittings.
There must also be a dielectric union or bronze nipple separating gas lines below grade from gas lines above grade and the above grade gas lines MUST BE bonded to the house electrical grounding system.
Some codes prohibit iron pipe below grade, and in those instances where it may be used below grade the pipe must be factory wrapped and all fittings must be field wrapped to prevent direct contact with earth.
All pipe used for gas service MUST BE NEW or previously used for gas service only.
Defective gas pipe or fittings must be replaced. It may not be repaired.
Also, regardless of material, when a gas line is laid under a slab it is required to be run through a conduit made of schedule 40 iron pipe or PVC pipe at least two nominial trade sizes larger than the gas line. The conduit MUST BE SEALED on each end and it must have an atmospheric vent outside the structure and at least 4" above finished grade.