I think the technology for gas meters is moving rather more slowly than electricity meters. Of course, there’s one major problem with a gas meter - there’s no energy source to tap into to provide any intelligence (of the sort we’re used to dealing with!). So you don’t have the luxury of a flashing LED to put a sensor in front of.
To the best of my knowledge, gas meters either have a rotating magnet and/or a reflective patch on the fastest register, or nothing. It is possible to use either a magnetic reed switch or a Hall effect device to sense the magnet passing by, or a retro-reflective optical sensor to see the reflective patch; then providing the meter calibration is known, it’s possible to derive the gas consumption and hence energy usage.
If you have the “nothing” type of meter, I have seen attempts to recognise the dial pointers or digits, either by watching for the pointer to go past or, in one case, using a camera and image processing software to “read” the meter digits or dial positions.
I believe some, by no means all, gas meters have fittings for mounting a sensor that’s available from the manufacturer of the meter.
If there’s a mains power supply available close by the gas meter (or at least reachable by a thinnish cable), then the emonPi and a reed switch would provide a stand-alone solution. Recalibration would be a simple matter of altering the scale factor via a web browser. If no mains is available, then a battery-powered emonTH and the reed switch could transmit the meter pulses to your emonPi located where there is mains power.
But I can’t recollect seeing anything about