Mikey,
Just to add a comparison which may demonstrate the issue. Most of us have some electronic items (mobile phones, mp3 player's etc) that use plug-in power supplies to run or recharge the devices.
Many of these require different voltages to operate correctly. Some Nokia phones needed a 3.8V charger, others need 4.5V and some need 5V. Most of us would not dream of deliberately using the wrong charger even though the voltages are similar. We have an understanding that if we did use the wrong charger, it is likely the product wont work or may work incorrectly or it may be damaged.
There is a possibility that a product may appear to work when the wrong supply is used, but components may have their working life shortened, or the products output may be less good that it should be or in some cases the output may be unsafe even though it seems to be working correctly.
Some manufactures do arrange for the product to have a wide input range - some laptop computer power supplies are rated to work with mains supplies from 100Vac to 250V ac, giving them work wide compatibility, but you will only know by checking the data plate.
In all of the above cases, the only way to ensure correct operation is to check the data plates on the devices and power supplies to make sure you only use them in accordance with the manufactures instructions.
Gas appliances and gas supplies should be treated in exactly the same way.