Required by law. Ontario law requires this credential for the work. A contractor without it cannot legally do the job, whatever the quote says. Building Opportunities in the Skilled Trades Act, 2021: plumber (306A) is a compulsory trade.
This work usually needs a municipal permit. Unpermitted work has to be disclosed when you sell, and an insurer can decline a claim arising from it.
Ask before you hire
Are you a licensed plumber, and what is your certificate number?
Is the call-out fee credited against the work if I hire you?
Can you camera the line and show me the recording before quoting a dig?
What is the after-hours rate, and when does it start?
Ask for the camera footage. Sewer replacement is the most oversold job in the trade.
You asked for the trade Plumbing, and the words "gas range hookup", and a rating of 4.5 or better, which also excludes every business with no reviews. Of the 12 businesses listed, none match every one of those.
The cost band and the licence requirements above still apply. They are what you need to judge a quote from anywhere else: whether the price is in the range, and whether the person quoting is allowed to do the work at all.
Listings here are seed data for demonstration and are labelled as such on each profile. Cost bands are what GTA homeowners are typically quoted, not offers. Always confirm a licence number against the issuing registry yourself before work starts.