There are plastic lines approved for gas. I'm not sure what you mean by "poly tube", but if you're talking about polyethylene water line, fugetaboutit. It's definitely not approved.
Googling "polyethylene gas lines" gives quite a few hits for companies that sell approved plastic pipe. However, I think this product is intended for underground burial and is NOT approved for use indoors.
Personally, if this pipe is going to be exposed, I would go with black pipe. That way you'll never have to worry about accidentally damaging it. But if it's impractical to install black pipe, you might look into corrugated stainless steel tubing (CSST). You can use it indoors and it will be much easier to install than black pipe. I know that Lowes carries it, plus there are plenty of online sources.
The Gastite company site as a lot of information on PE and CSST gas pipe.