How to convince plumber that there is a gas leak?
The other answers here are good from a technical point of view about how to verify a leak, but to answer the question of &quot;how to convince a plumber&quot;, I'll second a comment by @manassehkatz: call the gas company.
Any time I've suspected a leak the gas/energy provider has been very quick to send someone out with their own tester (not just a soapy water check). They are quite literally professionals at detecting gas leaks.
If they find evidence of a gas leak, that should be reason enough for the plumber to re-check and redo the work.