Well, you would have to make some modifications.
Such as breaking the glass and gluing on the magnet.
But first just make sure nothing else is magnetic so that the needle gets stuck.
Then the hall sensor can be mounted on the outside.
I assume you already have a solution for HA keeping track of how many rolls of toilet paper are in the toilet and how much paper is on them, because that, and just about anything I could think off, would seem more useful 
That said, it shouldnt be too hard to make something yourself. Using an arduino / esp32 and a transducer sensor, something like this:
To integrate in to HA, you may want to look in to ESPhome.
In all fairness I can actually see a use case for this.
When you are done with the BBQ you sit down and eat.
Then the next time you bother with the BBQ is when you need it, so finding out the state at that time is too late.
When you are in the store just open the companion app and have a look at the reading.
Im not a big bbq-er, so what do I know, but I would assume if you use your bottles until they are empty, that you have 2 of them, one always being full, and if the other is empty, well, HA is not much help.
If you only use 1 and refill it before it is empty, well, do it when they are not critically low? Doesnt make much difference if you check that on the app or on the bottle.
Anyway, I dont object to mostly pointless home automation, Im quite guilty of doing it myself. Just though Id poke a bit of fun at it.
How about measuring the weight of the gas bottle?
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most gas tanks have an empty weight printed on them, simple use a load sensor hooked up to a 8266 or something… some basic calculations less the empty weight will yield the percentage
God idea, but since it is attached to the hose, and gets moved around (Attached underneath the grill), I don’t think I can rely on the weight alone.
When you say “attached under the grill” that sounds like the perfect point for a strain gauge to measure the weight to me. Go a photo?
Not exactly my setup, but close enough, my grill and bottle is just a bit bigger. But same complete concept.
Look like you could easily put a resistive pressure sensor under the gas bottle. Some ideas in this topic:
Thanks 
I will give it a good one of these days
Interested to see how you get on with this, I’ve been trying to think of a good way to monitor the gas bottle in the caravan.
Yep, you have the exact same problem as I do
YES, we can just open the box on a daily / weekly basis, and check the gauge, but we always forget… This was just a quick idea to solve a “1st world problem” 
Running out of gas in the middle of something…
since the gass flow is regulated couldnt you use one of those water flow sensors? its just a spinny thing that calculates revolutions… you will likely need to empty a bottle to calibrate the thing and splicing it into the tubing would be unsafe and likely leak resulting in a small fire
Nice idea, but the flow is only present, once gas is flowing. But I might look into water preasure sensor instead… If I can find one that is also airtight
Please use only flammable gas rated fittings and sensors. Or you could have a very bad time.
Yeah, I know. I got carried away 
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Truma do a Bluetooth gas level checker, says it uses ultrasound, I wonder how easy that would be to replicate.
https://www.truma.com/uk/en/products/truma-caravan-rv-gas-fittings/truma-levelcontrol.html
Can’t you place it on a smart scale? And measure the weight?
you can get a weight sensor and attach to a esp8266 nodemcu.