IANAL, I believe the whole idea behind GPL is to make sure GPL supersede what you might claim otherwise. So if the federal government can possibly make a derived work into public domain (or crown copyright if UK) instead of GPL, that's a loophole in GPL!
(I know this work is not GPL, but similar idea applies)
Public domain code ain't GPL-compatible though unless it's explicitly released under a "license" that functions as a public domain grant (e.g. CC0, 0-clause BSD, WTFPL, etc., though WTFPL has some legal wording issues IIRC), the reason for this being that not all jurisdictions recognize the concept of "public domain" and thus require explicit license terms.
I don't know any jurisdictions that don't recognize the concept of “public domain”, but there are some that define “public domain” differently than it is defined in the US.
In the US “public domain” means “without copyright”, while in some other countries it means “without authors rights” (authors rights are a superset of copyright). The problem here is that some authors rights (both in the US & in those countries) are “inalienable”, meaning they can't be given or taken away (although there might be exceptions listed in the law).
Public domain may be incorporated into a GPL project if it is relicensed at time of commit under GPL by a committer whose country honors public domain to the degree necessary, but it may not coexist in a dual-license scenario with GPL, and that commit’s relicensing has no implied effect on the public domain repository from which the content was drawn.
(I know this work is not GPL, but similar idea applies)