Yes, the GPL in general has been upheld in court. The gist is that the GPL is the only thing giving anyone any right at all to distribute code licensed under it

Yes, the GPL in general has been upheld in court. The gist is that the GPL is the only thing giving anyone any right at all to distribute code licensed under it. If you don’t follow its terms, you no longer have that permission. The question isn’t whether the GPL can restrict you from doing something. It’s just that in the absence of the GPL, you fall back to the default legal rights where you’re not allowed to make copies of that code.