I hope and suspect that the US courts would refuse to enforce the judgement against Project Gutenberg. A little bit of research shows that for a foreign judgement to be enforced, a country would need to have personal jurisdiction over the defendant.
That doesn't seem to be the case here, at least under the US law that would apply if the judgement was being enforced.
Given all the overreach of European governments over the past few years (VATMOSS, GDPR, and now this), I suspect the time is ripe for something like the SPEECH act, a law clarifies that when a US citizen makes something available on the US, that does does not subject them to the law of a non-US country - and that allows the US citizen to claim court costs if someone tries to register a judgement against them.