Please first read the official Gentoo wiki page on Nouveau.
Nouveau is now available from the stable branch of the portage tree. Enable DRM_NOUVEAU in your kernel and set VIDEO_CARDS="nouveau" in /etc/make.conf. For detailed instructions, see http://www.gentoo.org/doc/en/xorg-config.xml
Previously, the nouveau ebuilds where in the nouveau overlay, which is now unmaintained. Use the x11 overlay instead, which is also available via layman
- x11-base/nouveau-drm (from git)
- x11-drivers/xf86-video-nouveau (from git)
- x11-libs/libdrm (from git)
x11overlay also has a mesa git ebuild, which contains the gallium based nouveau 3d driver.