Kernel not recognizing your hardware? Problems with power management or PCMCIA? What hardware is compatible with Gentoo? See here. (Only for kernels supported by Gentoo.)
Have you tried to use x11-misc/bumblebee? I have no problems with it (ok - the virtualgl stopped working on multilib with steam so I switched to x11-misc/primus).
Regarding lack of /dev/nvidia0 - it would indicate that a) nvidia driver is not loaded or b) something's else is wrong with initialization. Could you check if nvidia module is loaded (lsmod | grep nvidia)[/url]?
I've probably left my head... somwhere. Please wait untill I find it.