Copyright is the protection of the expression of an idea, not the idea itself. When you write that short story, or take that picture, your expression of the idea has certain protections when it is created. Registering the copyright is not a condition for protections of copyright.

Copyright cannot be abandoned unless you explicitly release into public domain. Registration is only to put everyone on notice that you have created something and seek certain advantages from the government if someone takes your copyrighted work. It is easier to prove your case in court if you have registered with USPTO and are seeking to prevent someone from using your work (infringing). Essentially, registering bolsters your claim that the work is your expression and not the opposing parties.

IANAL, but IIRC, the GPL is a licence which you grant to your users, allowing the them the ability to copy/use your copyrighted material.

Which... again IIRC... means that if someone manages to get the GPL turned down in courts, they are REJECTING the licence you provided them and there for, they have no right to copy your copyrighted material. (Although... I dont know if this denies them USE of it. Considering they obtained THEIR copy legally.)