Can I fix a possible gas leak on a valve for a hot water system myself with Thread Seal Tape?
I don't think it's as dire as all the comments are making it out to be. Some people are just afraid of gas, just like some people are afraid of anything involving electricity.  If you think it's just a loose coupling, you can turn the gas off, unscrew the coupling, apply pipe dope (not tape) and re-attach the coupling and turn the gas back on. If it still smells you aren't any worse off than before minus a few bucks for the can of pipe dope.