I'd check with you local Bldg Dept on the use of copper for gas. Here in San Diego area copper is never used.
You cannot use black iron pipe outside. Inspector nailed me for an existing exterior exposed black iron nipple on a job. You need to run Galvinized exposed on the wall.
The best pipe to use under ground is Polyethelene. You'll have to get 2 transition risers and bury it with 18" of cover. The Poly pipe has to be fused with special tools but once in should last almost forever.
Epoxy Coated steel pipe is another option but you have to wrap all the joints. If not wrapped properly the steel can rot out.
Used to be able to use CPVC (yellow) but they outlawed it because the fittings were not CPVC.
You sholud also have a valve at or near point of connection to the house so you can isolate it if you have a problem. Unions are not allowed except near the connection to an appliance. The correct way to connect the ground line is with a left/right nipple & coupling.
I can go on & on.
I'm not sure about the copper but I think there is something that is added to natural gas that reacts with the copper. I'll check into it. That would be an easy way to go though.
Propane is a different story.
I see a lot of applications where copper tubing is used with propane
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