I have a Hamlin 55100 based gas monitor here waiting to be installed on a job of mine.
I first tested the sensor on the gas meter in question when I was there last, using a USB connected nano (5v). It worked perfectly ok, so I have tidied it up, added an RFM (with voltage divider level correction) and put it in a emonTH case, there is a socket nearby so need to consider batteries. I’m just waiting to get back on site to install it.
I also have the bits here to monitor my own gas supply as when I tested this one on my own gas meter, it also worked fine. This time though, as the gas meter is on the opposite side of the wall that that my elec monitoring Pi is mounted to, I will just drill a hole and to connect to it directly via a USB port on the Pi rather than RFM and power adapter.
I bought the sensors from Sprirotronics