I have an insert that we use as a wood burning fireplace. It would be fairly easy to run a natural gas line under the house to the location of the fireplace. I would like to convert the wood burning to a gas fireplace.
The plumbing is not that hard, but you'll need a new fireplace and a new stove.
See if it's legal to use flexible corrugated line where you live. It's covered in a yellow plastic jacket. Corrugated Stainless Steel Tubing (CSST).
It'll be MUCH easier to install, plus under a house where there is moisture it'll hold up better.