I believe you are mixing apples and oranges. If you take a piece of source code, and modify it, _you_ do not own the copyright on that code. And, in fact, your modifications to that code fall under the GPL as well. Which means that if you send this code to someone else, they also must comply with the GPL. And they can give that source code to anyone.
You don't own the copyright on the unmodified code but you do own the copyright on the code you wrote. Yes? And your modifications fall under the GPL *if* you release it to the public, yes? It states specifically that you can do whatever the hell you please if you aren't releasing it to the public.
That's a legal question that is beyond my pay grade. I am not sure I can take someone else's book, modify it, and then claim the copyright on what i added, which is sandwiched in with what they wrote.
But yes, you are free to do whatever you want with a GPL program, if you keep it private and do not distribute it in any form whatsoever. But once you distribute it, then you have to abide by the GPL completely for any source that was GPL and which you modified. It remains GPL source.
You may convey covered works to others for the sole purpose of having them make modifications exclusively for you, or provide you with facilities for running those works, provided that you comply with the terms of this License in conveying all material for which you do not control copyright.
Conveying under any other circumstances is permitted solely under the conditions stated below. ...
It seems straightforward. He's not releasing it, he's conveying it to testers who are providing facilities for testing, which means the pages of rules following do not apply. It's an extension of the "you can do whatever the hell you want if it's private" rule specifically to allow you to have people other than yourself make modifications for you or test it for you.
I disagree with that interpretation. You can not take a GPL program, modify it, then send it to me and request that I keep it secret, but use it in a public way so that others see the process, but not the code. NDA and GPL are completely incompatible.
And any tester that sends the modified source to somebody else would be infringing on HIS rights, since his source is *still private*. His modifications don't even have to be GPL since it's not released to the public.
You don't have to release something to the general public to have to comply with the GPL. If you send it to anyone, that "anyone" has to comply with the GPL and can therefore send the GPL software to anyone else he chooses. You can send a GPL program to a group of friends. But you can't then require that those friends not further distribute the software, because the GPL explicitly does not allow that restriction.
From the FAQ:
Who has the power to enforce the GPL?
Since the GPL is a copyright license, the copyright holders of the software are the ones who have the power to enforce the GPL.
Hasn't the copyright holder already given his blessing?
Actually the FSF is usually the one that instigates lawsuits regarding GPL, but I would think the original author could do so if he wants.