Since no one seems to have replied to your GNU licence question : the GNU licence is designed to permit and encourage exactly what you are doing. You are under no obligation to advise the person whose work you are adapting; the only obligation you are under is to make the source code available along with the program. Obviously, it is polite to inform the person who wrote the original, and of course, the original remains his copyright, though the new parts are your copyright. You must mention all the copyrights along the way - e.g., if someone takes your work and adapts it, he would need to mention the copyrights of both yourself and William.

That's the essence of it.

HTH

Dave.