I've built a setup where I've placed a reed switch near the water meter in my house. Every 0.5 litre the switch gets triggered and my Raspberry Pi does the rest of the work.
My gas meter also outputs a magnetic field. So I've also placed a reed switch on it but it never gets triggered. I've tested with a magnet and the reed switch is not broken. I can "see" the magnetic field when gas flows through the meter and I keep my smartphone with an app to view the field near it.
The producer of the gas meter does sell a product to do the same thing I want to achieve so it should be possible. But why buy a ready made thing if I can do it much cheaper :).
Might I just need a more sensitive reed switch and if so how can I judge which one I need? The specifics of the switch which I'm using can be found here: http://www.produktinfo.conrad.com/datenblaetter/500000-524999/506957-da-01-en-REEDSENSOR_MS_313_3.pdf