Well, this is what Linus (father of Linux) said (taken from wikipedia) :

In the "Notes for linux release 0.01", Torvalds lists the GNU software that is required to run Linux:[11]

Sadly, a kernel by itself gets you nowhere. To get a working system you need a shell, compilers, a library etc. These are separate parts and may be under a stricter (or even looser) copyright. Most of the tools used with linux are GNU software and are under the GNU copyleft. These tools aren't in the distribution - ask me (or GNU) for more info.[11]

In 1992, he suggested releasing the kernel under the GNU General Public License. He first announced this decision in the release notes of version 0.12.[12] In the middle of December 1992 he published version 0.99 using the GNU GPL.[13]

Linux and GNU developers worked to integrate GNU components with Linux to make a fully-functional and free operating system.[