Using 12v kit on 24v supply

If the detector *always* draws the same amount of current, this works.

Presumably this gas detector does something when it detects gas? In which case, its current consumption probably will not be constant but will rise, perhaps by a lot, when it detects something. Which may, if fed via a resistor and if the unit has an alarm sounder, cause it not to sound or to sound like an unhappy mouse rather than a very annoyed coypu..

A resistor as a voltage dropper is fine if the unit always draws the same current and the source voltage is always fixed or if the unit isn't that fussed what it gets.

Only use one if you are sure that the unit draws the same current under all conditions and the voltage source is constant.

In this case, I would suggest using an inexpensive three terminal regulator, with a couple of capacitors as in the manufacturer's application notes, or a resistor/zener diode combination.