This stuff sticks all into the threads and gets torn up into tons of little pieces, so if you ever unscrew the fittings you applied it to, it's nearly impossible to remove properly.  When you then screw things back together, perhaps with new tape on the male end which is easier to clean, the tiny pieces all get pushed out of the much harder to clear female end and foul the gas line, potentially damaging whatever appliances you have connected or clogging a filter at best.  I had a propane distribution arrangement constructed of several T's and such, and when I needed to remove one fitting for a change, this fact forced me to deconstruct the entire arrangement so that I could effectively get all the little bits and pieces out of the fittings.  What a pain!  I wasn't about to do that again, so this hit the trash and I picked up some liquid PTFE thread seal, which works MUCH better.  Not only do I not worry about contaminating the gas line now, but the liquid sealant helps lubricate the fittings making it possible/easier to tighten them down further.  Search YouTube for a comparison of PTFE tape versus liquid to see others in agreement before you buy this!