board in NM,
There are a lot of sealants available that will work well with natural gas. If you can find it in your area I will recommend "RECTORSEAL T plus 2". It is a teflon based sealant that is rated by all of the appropriate agencies for use on natural gas. Unlike some of the other sealants available for natural gas it can also be used on potable water. By purchasing this brand you can use what is left on other projects. Plus it cleans up easily from your hands and clothes and has a long shelf life.
Plumber Rick is correct that the stainless steel tubings available are fast and easy but I personally still like the old black pipe.
There is nothing wrong with the dish soap method of leak detection. Be sure to test the entire fitting as well as the joint because once in a great while you will find one with a sand hole. If practical, test the entire length of the pipe if you buy it from a discount joint. I have found splits in the cheaper pipe. After testing, wipe dry and apply a thin coat of your cutting oil from a rag to keep the joints and pipe from looking rusty from the dish soap. Wipe away all excess oil. PLEASE DO NOT USE A LIGHTER TO CHECK FOR LEAKS.
Be sure to follow the instructions on the sealant can or package of whatever you decide to purchase and be sure to size your pipe properly. Gas pipe is not all that hard to do but it is important that it be done properly so that your son does not wake up in a tree.
Work hard, Play hard, Sleep easy.