GNU is not Unix
The GNU operating system was initiated by Richard Stallman in 1983. The goal was to bring a wholly free software operating system into existence. Stallman wanted computer users to be free to study the source code of the software they use, share software with other people, modify the behavior of software, and publish their own modified versions of the software. This philosophy was later published as the GNU Manifesto in 1985.
It was decided that development would move forward and the resulting system would be compatible with Unix. At the time, Unix was already a popular proprietary operating system. The design of Unix was modular, so it could be re-implemented piece by piece.
Much of the needed software had to be written from scratch. However, existing compatible third-party free software components were also used, including the Mach microkernel that forms the basis of the GNU Mach core of GNU. Most of GNU has been written by volunteers - some in their spare time; some paid by companies, educational institutions, and other non-profit organizations. In 1985, Stallman set up the Free Software Foundation (FSF). In the late 1980s and 1990s, the FSF hired software developers to write the software needed for GNU.