GNU stands for “GNU’s Not Unix,” and it represents the work of the FreeSoftwareFoundation (FSF) and its founder RichardStallman. Part of the project to create a Unix that was FreeSoftware included a highly functional editor as well as some sort of Lisp system. While I don’t think that RMS had planned for Emacs to become the Lisp support for the GNU system, in fact it has pretty much been the only Lisp to be found in the implementation of the operating system known as GNU/Linux.
See EmacsHistory for its history.
FSF Emacs or GNU Emacs?
M-x version RET
GNU Emacs 24.3.1 (i686-apple-darwin, NS apple-appkit-1038.36) of 2013-03-13 on bob.porkrind.org
So now you know.
Also see: CategoryBuilding