GPL type and derivative licenses offer no real protection to the author unless the author is prepared and can afford to enforce it.
Minor point of information:
Not necessarily true...
You could always assign the copyright
on the code to the FSF. Since only the copyright holder can assert their rights under law, unless you have deep pockets it's generally best (assuming you're really serious about licensing under GPL and not just toying with the idea) to assign it to the Foundation. Once they have the legal right to take action, they're very good at pursuing violations of the GPL.
If Eric had done that instead of just announcing he was putting his version of Circle Dock out under GPL, none of this might have happened.
I wouldn't have kept people from requesting source or making derivative works based on your continuation of Eric's work. But it would have prevented these characters from using it as part of a closed commercial product and then defying anybody (other than the missing Eric) to do anything about it.
Some people may balk at signing over their copyright. But since releasing under GPL is basically non-rescindable, it shouldn't really matter if you understand what going that route means.