I'm sorry to disagree publicly, but Bart's answer, while excellent in many respects, seems wrong in one: you may (and in my opinion should) put a GPL header in the files you acquired under PD but are now redistributing as part of your project. If you choose not to, you will need to make it clear that they are public domain, by adding a comparable "public domain" header if it is not already clearly there. And please don't mix PD and GPL code inside a single file.
Let me take that in parts: firstly, the idea that you may include the PD content under the GPL's conditions. Some call this re-licensing of someone else's content, and think it's never permissible. I take a more nuanced view and say it's permissible, unless the terms under which you received that content forbid it (as they often do). In this case, you received the public domain content under the most elastic terms imaginable, and you are perfectly within your rights to add conditions to its onward distribution. Anyone who wants to use the original files without those conditions can get them from somewhere other than the inside of your project.
GPLv3 s5c obliges downstream recipients to distribute the entire package under GPLv3. But it adds that they may (though they are not obliged to) pass on additional permissions if they have received them. Since PD gives the same rights as GPLv3 but with less conditions attached, it seems to me that PD status is an additional permission within the meaning of the clause. Distributing this content under PD is permitted, but not compulsory.
Why would I advocate changing the status? Simplicity. It's permitted, and it brings everything under a single licence. That in turn encourages reuse by making the licensing decision as to reuse as simple as possible. But if you decide not to do that, it's very important that you clarify the status of each non-GPLv3 file by clearly indicating that it's in the public domain.
