That would be a copyright violation, not a licensing issue. GP is giving you a _license_ to use his code without having to say "GP wrote this", he is NOT giving up his rights to the code..

edit: to clarify, he's allowing you to _use_ it however you like, including making a derivative work, including it wholesale, etc. However, claiming authorship of the original code would still run afoul of the original copyright.

edit2: oh - if you mean relicensing the code - that is allowed.