The Smappee uses a clamp to monitor electricity usage and an optical or magnetic sensor to monitor gas usage.
The Smappee gas sensor will not currently work with a SMETS meter. (They are aware of this.)
Based on the instructions for setting this new Bulb integration set up it seems the way it works is to utilise the fact that the Bulb IHD (In Home Display) is also a CAD (Consumer Access Device). A CAD device uses Zigbee to talk to the SMETS meter and then translates the data in to a form that can be accessed over IP, in this case WiFi.
This method can provide access to gas usage as well as electricity usage although the Smartthings documentation does not confirm their integration supports this.
Furthermore since IHD and CAD are standard functions although each CAD will have its own API this approach should be easily extensible to other brands. (Should Smartthings be willing to do so.)
I managed to identify the manufacturer of Bulb’s IHD6 and IHD3 devices. These are made by Chameleon.
At least one other energy supplier seems to use Chameleon which is Ovo Energy which means this approach might already work with Ovo Energy today as well.
Note: The fact that this integration is using the CAD method means it is not tied to a specific brand of SMETS2 meter only at the moment a specific brand of CAD.