So the "Recht auf das wissenschaftliche Kleinzitat" allows a European author
to "quote" small portions of e.g. GPL code without asking the author for
permissions. The European "Urheberrecht" on the other side forbids a minor
contributor to govern the license for the project that makes use of the
"Recht auf das wissenschaftliche Kleinzitat".
I'm not sure whether his conclusion is complete. According to http://www.sakowski.de/skripte/urheber2.html, citing is not allowed to replace (only to illustrate or backup) own statements of the author. I would therefore presume that whilst "quoting" GPL code, e.g., in a comment, to illustrate ones own approach might be OK, using GPL code as a mandatory element in the software would not be OK. But I am not a lawyer ...