Check with your local bldg. dept. to see what they accept. Is CSST run outdoors above ground a possibility with you and the AHJ? See if they'll allow you to paint the plastic covering or cover it with camouflage such as conduit or rain gutter.
Gas co use "plastic" for lack of a more descriptive term. In the "old" days we did use black pipe, however it was wrapped with muslin and tar on the job site. Man was that a fun job in the summer. Next came black pipe with a factory wrap,but joints had to be manually wrapped. Direct burial black pipe won't last long. Have seen leaks occur in as short as 1-2 years.
Well I just found out, too late mind you, that my power company Consumers Power in Michigan uses Polyethylene gas pipe underground and will make up a section of pipe for homeowners to be installed by homeowners for specific outdoor projects. I just installed copper so I'm a bit late.
In Lewisburg TN the gas co came in ~1988 & pulled PE inside existing black pipe -- clear to my meter -- had to give permission to dig out 90 elbow which was then cut away so pipe could gently curve around to make that bend.
I used to work as a Gas Fitter for a utility company. This utility had used all kinds of pipe over the previous hundred plus years, beginning with bored out Douglas Fir logs.
Black iron pipes could still be found in service decades after they were installed ---sort of. Sometimes the pipe was corroded away but the hole was still intact and providing service.
So black iron or galvanized pipe underground? No --- don't use it.
Steel pipe wrapped with tar and plastic is still used, but must be protected by cathodic protection of one kind or another --- not practical for the homeowner.
PE pipe is excellent and is what I would recommend. But this is not a DIY job. Have a competent gas fitter install the pipe.
Underground gas leaks are the most likely way to blow you house off its foundation. A leak from a gas service tends to saturate the ground, saturate the basement or crawl space and then has the explosive power needed to blow a house up if it gets touched off. And gas passing through some kinds of earth filters out the odorent added to natural gas, so there is nothing to smell.