GNU is a project and a Linux operating system.
"The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. Variants of the GNU operating system, which use the kernel called Linux, are now widely used; though these systems are often referred to as 'Linux', they are more accurately called GNU/Linux systems.
"GNU is a recursive acronym for 'GNU's Not Unix'; it is pronounced guh-noo, approximately like canoe." (from the website)
GNU has a site called "Savannah" which serves as a "central point for development, distribution and maintenance of GNU Software" at http://savannah.gnu.org/