I just did it.

Hi, I just hired a certified plumber to do it. He installed a gauge at the gas pipe then fill the pipe with air to detect the pressure of the pipe. If the pressure can be held at 10 ppm for 10 minutes, it means the gas pipes are good - no leaking. If the number dropped, the plumber will use the soap water to cover all the pipe joins to detect where the leaks are, then replace/repair it. I paid $500 for gauge installation and pipe repair. The price range can be 500 - 5000 depends where the leaks are.