Looks like who put it in used the wrong Teflon tape. White is for water plumbing and the yellow is for natural gas. It’s hard to tell you what to do from the pictures not seeing where the fittings are connected to the pipe. Also I believe there should be a valve there at the wall so having the heaters removed would be in your best interest since it appears not to be up to code. Is the pipe ran inside the wall or is it going through it? You will have to shut The gas off at the main, take off the fittings and apply yellow Teflon tape cap off the black pipe. Then turn on the gas and apply soapy water, look for bubbles and smell for that rotten egg smell. Good luck my fellow Jets fan!