GPL is here to make sure the source code doesn't stay secret, and it is an essential part of the Linux development process, the driver model in particular. In fact, I don't see how a community driven, widely supported, monolithic kernel could be made without a copyleft license.
Linux without GPL wouldn't be Linux, and out of the free OSes (BSD, GNU/HURD, Haiku, Minix, ...), Linux is the most successful.
By requiring the source to be available with the software, licenses drove the free software movement.