Here we suppose that your package is not FSF-copyrighted.
When you copy legally significant code from another free software package with a GPL-compatible license, you should look in the package’s records to find out the authors of the part you are copying, and list them as the contributors of the code that you copied. If all you did was copy it, not write it, then for copyright purposes you are not one of the contributors of this code.
If the code is supposed to be in the public domain, make sure that is really true: that all the authors of the code have disclaimed copyright interest. Then, when copying the new files into your project, add a brief note at the beginning of the files recording the authors, the public domain status, and anything else relevant.
On the other hand, when merging some public domain code into an existing file covered by the GPL (or LGPL or other free software license), there is no reason to indicate the pieces which are public domain. The notice saying that the whole file is under the GPL (or other license) is legally sufficient.
Using code that is not in the public domain, but rather released under a GPL-compatible free license, may require preserving copyright notices or other steps. Of course, you should follow the requirements stated.