1.a. Is it possible to copy-right not totally complete GDDs like mine which I described?
1.b. Would the copyright apply even after I make some changes to it?
2.a. For submissions to publishers, do publishers expect a complete GDD?
2.b. Or would it suffice to simply give them a document detailing the game concept and main features which distinguish it?
3. Publishers seem to be outside my country. Would my country's copyright laws apply if I send my documents outside my country to the publisher, who exists outside my country's boundaries?
1.a. Your design is automatically copyrighted. Read this: http://www.copyright.gov/help/faq/faq-general.html
1.b. Yes. Copyright is automatic.
2.a. No. They expect more than that. Read these:
2.b. No. That is not sufficient.
3. Yes. Your country's copyright laws apply worldwide, especially if your country is a signatory to the Berne Convention (Google it). And read those links I gave you.