The apartment buildings in my area all have wireless gas leak detectors, this video makes me want to take out my HackRF and start experimenting with mine.
Flipper Zero isn't what's causing this, the bad "smart" devices are, and the culprits themselves, of course. Just because you can hack a system, doesn't make it legal. Know your local laws. Mine says "accessing any IT system without authorisation is punishable with imprisonment from 3m to 3y; accessing the aforementioned system with the purpose of obtaining data is punishable with imprisonment from 6m to 5y.", vague enough for all of these things.
And reporting to the vendor is suicidal. At least assuming the stories I hear about vulnerability disclosures are representative, which I think they are.
In their place, if I were to inform the company, I'd do it anonymously. If it was an actually important issue - as this very much looks like - I'd consider informing the building manager, HOA, the gas installation company they use, and every local journalist, all together so they know about each other - and then CC that to the vendor.
Another option can be your country's CERT. In reasonably developed countries they generally have competent enough people to understand the concept of responsible disclosure (i.e. won't try to harass you for doing a good thing), and if they realize "oh shit, this is a critical infrastructure risk" they're probably in the best position to address not just the specific case, but also drive improvements (including via regulation) across vendors.
Flipper Zero isn't what's causing this, the bad "smart" devices are, and the culprits themselves, of course. Just because you can hack a system, doesn't make it legal. Know your local laws. Mine says "accessing any IT system without authorisation is punishable with imprisonment from 3m to 3y; accessing the aforementioned system with the purpose of obtaining data is punishable with imprisonment from 6m to 5y.", vague enough for all of these things.