There is plenty of information about Gentoo (Thanks God!), I am going to write here a very little how-to install Gentoo, then X, the Nvidia drivers, and Fluxbox, I will not go into details about Gentoo installation as there is enough in the official page.

Once you have your Gentoo Linux running, you can go to install nvidia-drivers, but first lets install sudo -Optional- (I like that sice I have used Ubuntu)

emerge sudo

Now edit the sudoers file using the command:

visudo

And enter this line at the end of the file.

your_user ALL=(ALL) ALL

Now continue with the installation

sudo emerge nvidia-drivers

Then load the module into your Kernel,

sudo modprobe nvidia

sudo update-modules

Then install X

sudo emerge xorg-x11

Now configure X

sudo xorgconfig

Follow the instruction on the screen, be sure to know the vertical and horizontal sync rates for your monitor.

Finally install fluxbox

sudo emerge fluxbox

Configure it to be your windows manager

echo "exec startfluxbox" > $HOME/.xinitrc

And start it:

startx

I got this error at this stage:

(EE) NVIDIA(0): Failed to initialize the GLX module; please check in your X
(EE) NVIDIA(0): log file that the GLX module has been loaded in your X
(EE) NVIDIA(0): server, and that the module is the NVIDIA GLX module. If
(EE) NVIDIA(0): you continue to encounter problems, Please try
(EE) NVIDIA(0): reinstalling the NVIDIA driver.

After googling, I found that the problem was that the GLX module loaded was the X.org module and not the nvidia one, so, solve the problem was as easy as enter this at command prompt.

sudo eselect opengl set nvidia

Now I have my first Gentoo with fluxbox on it running!!

Documentation

Nvidia on Gentoo Nvidia forum