Linus Torvalds states that without the GNU software from the Free Software Foundation such as the GNU C compiler it would not have been possible to develop the Linux kernel or most of the "open source" programs. The combination of the GNU operating system and the Linux kernel make up what is called the GNU/Linux operating system. Torvalds also states that he agrees and uses the GNU General Public License (GPL).
To learn about GNU visit: http://www.gnu.org
The Free Software Foundation: http://www.fsf.org