don't think you can GPL this sig, unless you've got the copyright on it.
Well, can they publish it under GNU Free Documentation License?
the point is, no one but the original author can do that. if it is in public domain, then it must remain so. the GPL or GFDL are more restrictive than public domain. of course, now i'm starting to drag levels.
IMHO if its public domain, you /can/ release it under any license you wish, including one which says everyone who reads this sig must pay me a million bucks. However such relicensing is not likely to be effective since one can just obtaion the stuff from another source.
Public domain means you can do pretty much anything you like with it, except perhaps claim that you are the original author. That includes releasing it under a more restrictive license. It was precisely to prevent this that GPL & Co. were drawn up.