Meanwhile I found this great resource on github:
https://github.com/sarnau/MMMMobileAlertsIt provides documentation about the ELV Mobile Alerts gateway and sensors.
And it says that it's "a version of the LaCrosse Alerts Mobile system made for the European market".
"Despite looking almost identical and the expectation that there are only subtile differences in the sensors, like the operating frequency (915MHz in the US vs 868MHz in Germany), the protocol seems to be completely different. Anything on the internet about the La Crosse Gateway GW-1000U, which looks identical to the Mobile Alerts Gateway is not applicable. There is a certain overlap, but it is minimal (like the UDP communication on port 8003). The sending is different, e.g. the http_identify does exist, but the format is different."
On one hand, it's a disappointing to know it's not the same device and I can't use resources available for the GW-1000U. But on the other hand, it should be possible to use it with weewx. Will be hard work to get the setup running...