Revert is not the correct term. GPL-licensed code is still and always copyrighted, even when GPL-licensed. The GPL is itself a copyright license.
Only CC0/WTFPL/Public Domain are renunciations of copyright.
Only CC0/WTFPL/Public Domain are renunciations of copyright.
Also, when it comes to code dedicated to public domain it is good to consider what wikipedia says about implied warranty: "The licensing process also allows authors, particularly software authors, the opportunity to explicitly deny any implied warranty that might give someone a basis for legal action against them." It is always good to include a clause saying that there is no warranty, implied or otherwise (unless of course you provide a warranty, which you probably should not do without restrictions on OSS code).
They say it's compatible, but not recommended.
"Unlike the Public Domain Mark, CC0 should not be used to mark works already free of known copyright and database restrictions and in the public domain throughout the world. However, it can be used to waive copyright and database rights to the extent you may have these rights in your work under the laws of at least one jurisdiction, even if your work is free of restrictions in others. Doing so clarifies the status of your work unambiguously worldwide and facilitates reuse."
As I understand it, you can't just waive copyright on your own work - copyright is something you get automatically in most jurisdictions on the planet. Licenses like CC0 and WTFPL are operating within the copyright regime - they use the laws to make your work behave as if it was not subject to copyright - but they can't actually make your work not copyrighted.
It's like a difference between simulation and reality. CC0, WTFPL, etc. are simulating a copyright-free reality on top of the copyright system. No matter how close to perfect they get, it's still a simulation, and copyright is the underlying runtime.