In my state [New Jersey] you need to be a licensed plumber to do gas lines. Or the homeowner. If it's above ground, I can file the permit as an exempt contractor, and if the town ok's it, I can do it. But the town has say-so.
Copper can be a bad idea for natural gas, depending on how clean or dirty it is. Plus the joint from steel to copper can be a problem. Around here if it's going in the ground, it's either steel pipe with factory applied coating, or fused polyethylene with tracer wire and warning tape.
Being a certified pipe fitter is only as meaningful as the regulating body that certifies him.
Also, the town should have ultimate say on type of pipe. Size would also need to be calculated, not guessed at.
The PE pipe is less expensive and easy if you have the right tools and are factory trained.
But check with the town and make sure a licensed plumber isn't required.
Late post, just to clarify some things said in this thread in case others use it for reference.
Correction note: In New Jersey, any person can install gas pipe. No license, certification or otherwise is needed.
But to connect the piping to the gas source (such as gas company meter), or to connect any gas appliance (such as furnace, water heater, stove, etc.) to the piping, you need to be a Licensed Master Plumber, or be the home owner.
Also, using plain black pipe outside is not recommended because it will rust. Either coat the pipe after installation with galvanized paint, or using galvanized piping instead of black. Painting will most likely be cheaper, but you need to make sure every bit of the black pipe is completely coated with the paint.
There is absolutely nothing wrong with poly gas pipe. Short of a manufacturer defect, it is the best way to go. Currently it is estimated to last 100 years in some brands.
The water pipe issue was due to the chlorine added to the water "eating away" at the plastic water pipe causing a toxic mixture in the water. Gas lines do not have chlorine and only small amounts of water, so this is not an issue.
