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.

I'm not suggesting that you avoid following it. I'm just not that convinced it's enforceable in the US. The French ruling is good, though.