Past years---12" minimum Clear-Outside to Outside was sufficient.
However, we have encountered recently, this last year, for major gas lines, (So. CA. Gas), 18".
Best to ask an Engineer at the Gas Company.
Some of the agencies have required wrapping the D.I. or enclosing and sealing (encasing) the piping in a secondary containment.
Wendell Iwatsuru, P.E.
Madole & Associates, Inc.
9302 Pittsburgh Avenue, Suite 230
Rancho Cucamonga, CA. 91730
In San Bernardino county, I have seen gas companies requesting 12" vertical separation for standard distribution gas lines and as much as 24" vertical separation for high pressure gas transmission lines. For standard gas distribution lines, I have experienced little resistance when requesting a variance for 9" vertical separation or as little as 6" vertical separation when it avoids an offset in either line. However such a request must be made directly to the gas company's planning department which represents your jurisdictional boundaries (in my region the SCG transmission lines are represented by a different planning/design group than the distribution lines). These contacts can be easily found through digalert design lookup. They typically formally request 8 to 12 weeks for plan review of a 90% design set, but of course you want to know if a variance is acceptable much sooner than a 90% set and they typically are agreeable to advise as soon as you have a plan+profile for the crossing. Good luck with your project.