Getting close. Canada Post screwed up again. This has happened to me more times than is reasonable. Should be here tomorrow, but who knows?! 
So maybe the simple answer is a combo sensor/safe exhaust fan? I am thinking like the common garage opener & door tilt combo...
We could mount a gas sensor low (gas heavier than common air) in the garage, crawlspace, basement, attic, and or kitchen.
We could then mount the safe exhaust fan to the existing exhaust pipes for the water heater, the fireplace flue, kitchen hood, or even a bathroom fan.
How many DIY smoke detectors and fans get put up every year?
Seems like cheap insurance versus loss of homes and/or lives!
I'm thinking it should be mounted high since natural gas rises, and that's the primary gas most people are using. How fast it rises I don't know... ie: I don't think a gas leak in the basement would race to the third floor... I think it would concentrate on the basement ceiling. (Eventually)
Explosion proof fans are typically found in industrial settings. They are very expensive. An affordable best bet for action upon alarm is closing the source via a professionally installed electric gas valve.
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Do any connected gas valves exist yet?
Need an outdoor shut-off valve at the meter. No risk there. Only thing is how to power such valve. Battery?
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Might be an opportunity. Lowes, Home Depot, etc. sell millions of small DIY bath exhaust fans every year. Just upgrade the fan motor....like all of those brushless drills on sale now...
Easy: Gas powered. Gas burns a flame which spins a motor which generates a current which powers the device. Voila!
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I like where you are going with this. But no flame needed. Use gas flow to spin a turbine to generate power.
would be good to turn it off, but we'd have to go outside to manually turn it back on.
I wonder if you could use something like this (easy to make your own) and a basic water/gas valve like the Dome and put them on your outside gas valve to turn it on/off.
Where I live, Minnesota, gas meters are indoors. While it might be nice to have a shutoff valve before the meter, the gas company shutoff valve is quite stiff. The ball valve just beyond the meter, for me, is very similar to a water shutoff ball valve and will work with one of the water shutoffs. I do use a water shutoff on the gas feed to my backyard kitchen. It works quite well. Once I know there is a working detector, I will get one and another water shutoff.
Christmas in July! My Aqara Cube showed up from an Ebay seller too.
Those familiar with the size of the Aqara cube will be raising an eyebrow right about now.
Xiaomi has this knack of constantly shocking me with the size of their devices. It's like their world is so much smaller than ours! 
And if there was any question about certification, it is answered here. It is NOT certified for use outside of Mainland China. So you buy this, don't expect any compensation from your insurance company or a blessing from your local fire department.
My personal statement on this device:
You purchase and use this device, you do so of your own accord and accept any and all associated and potential risks. From this point forward, I do not endorse or recommend it in any way. You are 100% liable for the use and setup of this device. I hold no responsibility for any mishaps, incidents, bodily harm or deaths relating to, or resulting from the installation and use of this uncertified device, which clearly states on the back of the device in English, that it is Only for Mainland China
OK, now, with that out of the way, @veeceeoh here's the fingerprint you need to get started. 
Thank you so much for volunteering your time with this.
Great! I actually already have a preliminary beta driver almost ready to share.
However when I looked the device capabilities list in Hubitat developers documentation, I don’t see Gas Detector. So the driver will be using Smoke Detector for now, instead of a custom attribute.
I’ll be posting with a link sometime today.
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Beta v0.1b of the Xiaomi Mijia Gas Detector driver is ready to test!
Crossing fingers that this works even just in a basic way...
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You could install something like this
and a Z wave or Zigbee relay to control it.
Nice.. now get to work and fix my driver 
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Is your driver the Orbivo one from the ST forum? Not seeing where you posted it.
It pairs fine as Device. Using the Orbivo driver with physicalgraph changed doesn't yield much:
I'm thinking it might have to do with:
import hubitat.zigbee.clusters.iaszone.ZoneStatus
import hubitat.zigbee.zcl.DataType
Things of note right off the bat: For natural gas, which most of us will be using it for, they suggest mounting up high, 0.3m-1m from the ceiling. Above the stove would be my first thought. Unfortunately I don't have an outlet up there.
Also, it has a 3-minute warmup time after powering on and resetting, so grab a drink, kick up your feet and relax.
I believe those lines aren't necessary in a Hubitat driver.
