NOTICE: Not A duplicate of Am I allowed to add additional copyright on modified code that originally the FSF released under GPLv3: The previous question answered if I'm allowed to copyright modifications to software published by the FSF at all. The current question tries to find out if the FSF applies the same rules regarding copyright to forking as they do to contributing.
I've been reading the GNU FAQ and found the following:
Why does the FSF require that contributors to FSF-copyrighted programs assign copyright to the FSF? If I hold copyright on a GPLed program, should I do this, too? If so, how? (#AssignCopyright)
Our lawyers have told us that to be in the best position to enforce the GPL in court against violators, we should keep the copyright status of the program as simple as possible. We do this by asking each contributor to either assign the copyright on contributions to the FSF, or disclaim copyright on contributions.
We also ask individual contributors to get copyright disclaimers from their employers (if any) so that we can be sure those employers won't claim to own the contributions.
Of course, if all the contributors put their code in the public domain, there is no copyright with which to enforce the GPL. So we encourage people to assign copyright on large code contributions, and only put small changes in the public domain.
If you want to make an effort to enforce the GPL on your program, it is probably a good idea for you to follow a similar policy. Please contact firstname.lastname@example.org if you want more information.
From my understanding I must disclaim copyright or assign it to the FSF if I'm contributing to FSF-copyrighted programs. Does the same apply if I fork the project in order to modify it and port it to another compiler?