layman takes care of it, so if you cat its make.conf you get this:
ocalhost cvill64 # cat /usr/portage/local/layman/make.conf
PORTDIR_OVERLAY="$PORTDIR_OVERLAY
/usr/portage/local/layman/sabayon
/usr/portage/local/layman/gentopia
/usr/portage/local/layman/science
/usr/portage/local/layman/portage-xgl
/usr/portage/local/layman/nx"
So /usr/portage/local/layman/sabayon
localhost cvill64 # ls /usr/portage/local/layman/sabayon
app-emulation media-plugins net-wireless sys-kernel x11-misc
dev-python media-sound sci-biology x11-base x11-themes
gnome-extra media-video sci-misc x11-drivers
media-libs net-misc sys-boot x11-libs
There you go
