Couldn't you calculate this simply by monitoring the "run time" of the devices using the gas? If we can tell how long the furnace and/or water heater run, then you can do the math vs. the gas bill?
Now that's a good idea! I could tell whether our central heating boiler is running by its electricity consumption (which will either be on/off) and measure what it actually consumes when it is on.
Kind of obvious once you mention it - as all the best ideas are!
I've thought about it quite a bit, it wasn't off the top of my head; I would also like some sort of usage monitoring, but I'm not eager to tinker with gas lines or the insides of a furnace.