Could be a gas line but not supposed to be, check the code’s to see whats recommended. Galvanize nipples also come in several sizes, meaning the length and diameter. Inside dimensions in reference here so be sure before purchasing. And don’t forget the pipe thread tape wrapped around before assemblage, this insures a seal. Important not to mix metals here folks, electrolysis is a  major foul, copper,brass and stainless is good, but brass and galvanize is bad, instant metal wrought for future problems. Pipe thread is the basic means of thread count in this field but is paramount to get it right the first time in DIY plumbing repairs. PEX plumbing, PEX fittings, Do It Yourself with PEX

PEX (or crosslinked polyethylene) is part of a water supply piping system that has several advantages over metal pipe (copper, iron, lead) or rigid plastic pipe (PVC, CPVC, ABS) systems. It is flexible, resistant to scale and chlorine, doesn’t corrode or develop pinholes, is faster to install than metal or rigid plastic, and has fewer connections and fittings.

TRY PEX Tubing and fittings:

PEX fittings are generally made of brass, although some vendors are offering bronze, copper and engineered plastic fittings for PEX. The characteristic ridges on the “insert” part of the fitting distinguish a PEX fitting from other fittings. The ridges, the PEX tube and the crimped copper ring all work together to form a high-pressure seal.

Typical Galvanize Pipe failure for weak water pressure! Undo with a pipe wrench and channel locks by grabbing the pipe at nearest fitting, spin off and out to replace. Nipples come in any circumference and length, pre-cut and threaded ( inside diameter) not outside!
Don’t forget the teflon tape!
Black Steel GAS Pipe: Typical gas pipe pre-cut parts and not to be interchangeable with regular galvanized pipe. Mostly for older repairs or replacement parts.NOTE: Check all fittings with soapy water, before mission’s accomplished! Special tape for GAS pipe needed, ask for it!