If you are writing code that will eventually end up in a GNU project such as GnuEmacs, then you will have to worry about legal matters: You will need to sign a copyright assignment to the FSF. The FSF then has the copyright over your work. This is different from releasing code under the GPL. You can always release code under the GPL without assigning the copyright to the FSF. The only effect this will have is that your code cannot be part of GnuEmacs. It can be part of XEmacs (XEmacs), however.
If you are writing a patch to code that is part of a GNU project such as GnuEmacs, then you will have to assign the copyright as well, if your patch is “significant”. For the GNU project, any patch spanning more than 10 lines is considered “significant”.
The section about Copyright Papers is the one where they talk about the “assignment” stuff (“Papierkrieg”):
Training for your lawyer skills: