I've had 3 of an earlier version from Lidl on the boat for 1-2 yrs and seem fine; these were a self contained unit with sensors below the unit box. about 2-£3 as I recall.
If the gas alarm is the same as the one they were selling last year it's actually 12V with a transformer. So just cut the transformer off and wire it up to 12V. I did this on my boat and it works a treat