Just to explain the history. The OS is GNU (GNU Not Unix), which was a set of libraries developed by Richard Stallman in the early 80's for his Hurd OS. The one thing he tried to create and needed to run Hurd was a kernel.
Along comes Linus Torvalds who was studying Minix, and in 1990 he had developed the Linux kernel. Stallman quickly put this to use on his project and GNU/Linux was born. No GNU, no Linux. You can't have one without the other.
Side note. There are also parts of BSD included too!