That is the Gambelli ruling. One should be able to search for an English language version, it is quite famous and emphasizes the cooperation of the European Union on this. Gambelli is really an Italian case.
All kinds of European Countries added their opinions to this case as it was being tried.
It is illegal to play if you are a German residing in Germany, or to host anything to do with online gambling, or to advertise it.
Germany has a couple of huge land casinos that pay taxes through the nose. It makes sense for the Germans to try to keep all gambling confined to these so the money flows into the government koffers.
As usually, it’s the money.
I have seen the German tax officials go after people living in the states.
I have not heard of any Germans in gaming being arrested yet, and I do know of a few.