Depends on your purpose with the gas line:
For design, use
ASME B31.3 for Process Piping
ASME B31.2 for Fuel Gas Piping
ASME B31.8 for Gas Transmission ans distribution (Pipelines)
B31.3 is the normal code for refineries, treatment plants, offshore platforms etc.
If you're sub-sea I'm not sure whether B31.8 cover, and you may need another standard.
For components use
ASME B16.5 for flanges
ASME B16.9 for BW fittings
ASME B16.10 for valves
ASME B16.11 for SW firrings
ASME B36.10 for pipes
And possibly other codes referenced in the above.
Generally these standards were previously known as ANSI standards but now they are all designated ASME.