If I ask "Is a contract enforceable", it means "can one party to the contract sue someone else under it?". The GPL is not a contract . It's a bare licence: it grants the licensee permission to do something that would otherwise be illegal, but it places no obligations on that licensee.
You cannot sue someone (whether you are licensor or licensee) under the GPL, so the question "is the GPL enforceable?" is meaningless. The GPL has conditions, and if you don't comply with the conditions, then you're outside the scope of the licence, and not covered by it.
You can sue someone for infringement of copyright. If you're the owner of the copyright in software which you've released under the GPL, and someone is, for example, distributing that software outside the scope of the GPL, then they are infringing copyright because they are going outside the scope of licence granted.
The copyright owner sues them for copyright infringement. The alleged infringer can then invoke the GPL and try to argue that he or she was operating within its scope, but like any bare licence, the GPL can only act as a shield against a claim, not as a sword allowing anyone to make a claim in the first place.
So if want to twist the meaning of the question so that it means something, that something would have to be: "Is the GPL effective as a shield?", This is important, because by asking the question "is the GPL enforceable" people tend to think that if the answer is "no", then the software is essentially in the public domain, and they can do whatever they like with it with no restriction whatever.
Clearly, if the answer to "is the GPL effective as a shield?" is no, then the upshot is that you can never make use of the software without infringing copyright, because if the GPL is the only thing stopping your use from being infringing, and it's not effective as a shield, then whatever you do (subject to fair dealing....etc.) will be infringing.
Imagine a simple licence: "You can only use this software on Tuesday". Is this licence enforceable? What does the question mean?
The owner can never sue me under the licence: if I'm using the software on a Wednesday, and get sued, then clearly I will fail if I attempt to use the software as a shield, but if I use the software on a Tuesday, and the owner tries to sue me, then I will point to the licence and say "but I have permission to use the software on Tuesdays". In either case, the owner will be suing me for breach of copyright, not for breach of the licence.
Like everything else in the law, it's not necessarily quite a simple as that. There are some commentators who argue that the GPL (and other free and open source licences) are contracts and not bare licences.
I won't go into these here, but I'll stand my corner, and say that in the English legal system, the GPL is a bare licence (as it happens, this argument doesn't hold at all in many continental jurisdictions which use the civil law system). Also, the licences under which you use proprietary software (and which compel you to pay) are, generally speaking contracts.
There are some legal consequences of the distinction which are reasonably esoteric (and may be important in certain circumstances). They include how damages are calculated, whether the licensee can be forced to do certain things, and the availability of injunctions. The differences, are however important.
To look at it another way, is the question "is a driving licence enforceable?" meaningful? You're not allowed to drive a motor vehicle on the road without a licence. Can you enforce the driving licence against someone? The police? The highways agency? No.
The only thing that a licence permits you to do is, in practice, to defend yourself against a claim that you are driving without a driving licence. Like the GPL, a driving licence is subject to conditions and limited in scope: it may well be restricted to motorcycles, or cars with automatic transmissions, and it will expire on your 70th birthday unless you renew it.
The idea of a licence which is subject to conditions is easy to grasp when it's a driving licence. For some reason, people can't grasp the same point when it comes to the GPL.