Gas lines are made with black pipe which is available at any home improvement store; some will even cut to your request and thread it for you.
There will always be a shut off where each gas line comes up through the floor leading to an appliance. Just past the shut off a flexible line will be attached which actually connects the appliance to the gas line.
Gas leaks are a major concern and all joints need to be tested with soapy water to make sure there are no leaks. It is possible for the flex lines to crack too, so be alert for the smell of gas ( actually the gas doesn’t smell, they add a chemical closely related to skunk spray to it). If major changes or repairs are being made to the lines themselves the system needs to be pressure tested before use. This involves pumping air into the lines to 14-15 lbs. pressure and then watching a pressure gauge to see if it drops.
The gas supply to all appliances must be turned off before the pressure is applied because the regulators in the appliances are not designed to handle such high pressure and may be damaged. Besides they will probably leak and thus make the system fail the pressure test.
I was once told by a licensed, bonded, insured plumbing and heating company that my house had failed the pressure test and all the gas lines would have to be replaced; when the problem was they had not noticed a gas dryer and shut off the gas to it.
At the factory the guy building gas lines knows the floor plan by heart and the lines are built in one area and carried as a unit to the installation area. That means that he doesn’t have to worry about things like unions (pipe not labor). He has all the room he needs to work and is never going to have take anything apart. What does he worry about? He wants to be sure there are no leaks during the final factory testing because then he would have to go find and fix them.So is there any chance they might be overtightened and difficult to disassemble?
When someone says to you, as a guy did to me, “could you run a gas line over to the laundry area so we could use our perfectly good gas dryer in this house?” don’t be to quick to say yes. It sounds so simple, unless you know it means you will have to cut the line, remove the cut pieces and have them threaded, shorten them a little to allow for the T you will have to put in and then pressure test. During the pressure test you are very likely to discover that in the process of cutting and removing the pipe you managed to create leaks farther down the line. I spent almost two days on this “simple” little favor and had to replace half the gas line in the house.