GNU is an operating system that is free software—that is, it respects users’ freedom. The GNU operating system consists of GNU packages (programs specifically released by the GNU Project) as well as free software released by third parties. The development of GNU made it possible to use a computer without software that would trample your freedom.
And btw an OS is more than a kernel, the kernel is „just“ the foundation on witch the OS works. Hardware communication is on kernel level, for example.