It really depends what are connection options for your gateway. There are multiple smart meters and metering standards (m-bus, wireless m-bus, cosem, country specific meter protocols). Some of them are supported directly in openHAB, some come in third party addons, some are not supported at all.
I did for example implement integration for Relay Weblog gateway for our customer cause he plan to have it in several installations.
If you have an interest in support for a specific hardware then you can start from collecting materials on how to “talk” to gateway or meter so others will be able to advice you how to approach whole thing. For example you could try to scrap data over http from admin interface of gateway (if it shows most recent read outs).
According to this presentation:
There is an working integration. Not sure if its coming from vendor or someone else.
One more material I found is in german, but I can’t judge if it does answer any questions: https://www.ppc-ag.de/wp-content/uploads/media/PPC_Smart-Meter-Gateway_Handbuch-für-Verbraucher_v.4.1.pdf