Oh FFS.... Please, not the fsckin "IT'S GNU/LINUX!!!111" argument again *sigh*
As a certain advertising campaign says, let's get the facts.
1. By the time the Linux kernel was actually useable (mid 90's) much of the core GNU project was in a mature state. There was a full ten years between the beggining of the GNU project and that of Linux.
2. Linux took the place that the HURD kernel was meant to take (HURD is the GNU project's own kernel), but, after 14 years in the making, HURD still isn't complete. The FLOSS community of the time needed a free kernel, and HURD was even more broken that it is today (yes, that is possible...) and Linux filled that need. This is when Linux began to "take off", as it were, as there was now a completely free OS. Linux kernel + GNU system.
3. Linux was and is developed using mainly GNU tools. The GNU project started in 1981 to replace the main set of standard proprietary UNIX tools of the time. Linux and apps are built with GCC and friends.
4. GNU was being used in many commercial and some free unicies a long time before Linux was around. You need a suite of tools before you can develop a kernel. GNU constituted much of that suite.
Soooo... GNU appeared first and was used to develop Linux. Linux then became a popular kernel and was distributed with the then already well-known set of GNU tools. This combination rapidly became very popular first in the community and by the late 90's in many servers (think Apache). This continues to the state Linux and GNU are in today, i.e. a commercial grade system deployed by many major businesses and organisations as a server and now even a desktop OS.
BUT, to people who bang on about "Linux made GNU possible" and "OMG!!! GNU WAS THERE FIRST LINUX WUOLDNT EXIST WITHOUT IT!!": Just stfu. They're both necessary components of an OS. Whatever the situation was in the past, just live with it. They're now mutually dependant and work well together so BE HAPPY.
Currently, GNU, Linux and apps are all just as important. The apps won't run without a kernel and system software, and a kernel and system software are useless without applications to run on them. You need a kernel (Linux), system software (GNU, etc.) and applications (Tremulous) to have a USEFUL WORKING SYSTEM.