I would not put any current on the line and just drive a ground rod/pipe. I doubt you would have an issue but sometimes you can get a reaction that can erode the pipe causing a leak. In this part of the country we get a lot of lightning strikes on our fences which also would not be good. A good ground rod or two and don't forget the lightning arrestors is what I recommend.

article 250.52(B)(1) specifically states you can not use a gas pipe as a grounding conductor.

"(B) Not Permitted for Use as Grounding Electrodes.
The following systems and materials shall not be used as
grounding electrodes:
(1) Metal underground gas piping systems"

Drive the ground rod.

BTW, I find that a steel fence post driver helps quite a bit when sinking a ground rod. Beats standing on a ladder with a sledge and a helper who's hoping you don't miss!