Can questions be asked on StackOverflow and then answers be contributed to projects licensed under the Gnu free documentation License (such as PLEAC).
All content is licensed under Creative Commons Attribution-Share Alike 2.5 Generic. So any usage of code from here is subject to that license.
IANAL, so grain of salt and all that.
I believe that you can (try to) contact the original poster to obtain permission to use it in a GPL context. Because the copyright holder can license the creation several different ways if they so desire.
If this sounds like the way you want to go, contact a IP lawyer.