I don't believe that they could successfully charge you with any offence based on an anonymous (=unverified) report provided to GGL.
Honestly, when I'm reading this from you, I have a strong feeling that GGL just tries to discover some new waters and they are trying to set the playing field since the gambling regulations are still a new concept in Germany. You've been unlucky since they targeted you for their experiment.
You sound like a reasonable person so I hope you'll manage to resolve it successfully and possibly set a new precedent for other German citizens.
There are companies that help players to get refunds from unlicensed casinos. I'm sure you must have them in Germany as well. If I was looking for a lawyer, I'd probably start with those companies since you really need someone who knows a lot about the new gambling related legislation.
Automatic translation: