There have been very few court cases that have said anything close to conclusive about whether the GNU General Public License is enforceable as a contract.  Christian Ahlert of the Oxford Internet Institute has translated
a District Court case from Munich which, in Ahlert’s view, is the first
case to determine that the GPL is enforceable.  Ahlert’s
commentary is here.  Prof. Dr. Thomas Hoeren adds more, by way of critique.

Progress Software Corporation et al. v. MySQL, albeit quite inconclusively, might be the closest US case to have done so to date.