The transition from metal to plastic (polyethelene)gas lines is an old story by now. All the pipe I had installed had been epoxy coated (green) pipe with pipe wrap at the corners. Since then, we have transitioned to yellow plastic pipe which is much faster to install, though the fittings are much more expensive.
Here is a short video on how to install an inline coupling with yellow plastic gas pipe.
THIS IS NOT FOR NEWBIES. Special tools are required and some training is needed.