There have been a number of of cases, which are linked from Wikipedia (https://en.wikipedia.org/wiki/GNU_General_Public_License#Leg...) - most recently Entr’Ouvert v. Orange had a strong judgement (under French law) in favour of the GPL.

Conversely, to my knowledge there has been no court decision that indicates that the GPL is _not_ enforceable. I think you might want to be more familiar with the area before you decide if it's legally questionable or not.