I have been setting up a monitor on my gas meter today, I have noticed there is a mirror spot on the “6” of the 3rd decimal (0.001) but I do not use use it, I have found there is also a magnet in the tumbler and have set up a hall sensor as mentioned in the "Hall sensor possibility for Gas Measurment?" thread. Although power consumption is not a primary concern for me as it’s not battery powered, the using an interrupt rather than constantly reading means the unit could be asleep most of the time.
Have you checked for a magnet? they are quite weak, but thier presence can be confirmed with a run of the mill old fashion magnetic orientation compass.
Only if the sensor will operate at 3.3v, a quick look at the page you linked says it is working at 5v but doesn’t seem to confirm if that has to be the case. The emonTh operates at 3.3v so it can only provide 3.3v at D2 despite there being 5v available at the terminal block.
EDIT - found a datasheet for the sensor, but I’m none the wiser 
http://www.vishay.com/docs/83760/tcrt5000.pdf