It's worth noting that neither the GPL nor Creative Commons are "public domain"; they are both forms of copyright. If you want to place a work into the public domain, simply put a statement in the front of it saying something like "This work is in the public domain".
Make sure that the public domain really is what you want, though. Anyone can do anything they want with a work that's in the public domain: modify it and re-publish it under their own name, commercially sell it - absolutely anything.
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