No matter what an expert's study on the case might look like, the proof of availability for a sufficient number of free licensed content to counter GEMA's presumption has to be brought forward to court. The "GEMA assumption" is part of German jurisdiction, therefore it's much likely the ruling has to be found at Germany's highest court, the Federal Court (BGH). But the true challenge is its verifiability.
Tag / collecting societies
I think it's worthwhile mentioning again that there are at least two significant differences etween GEMA and US collection societies: GEMA is the only collection society in Germany. Its factual monopoly is acknowledged. Members of GEMA strictly are not allowed to license some works with GEMA while others are licensed under Creative Commons for instance. All works you are creating during your membership solely are managed by GEMA.