Configure layman
apropriadamente para que o portage pesquise suas sobreposições adicionadas.
Option 1: Using 'make.conf' method (older method)
echo "source /var/lib/layman/make.conf" >> /etc/portage/make.conf
...
Option 2: Using 'repos.conf' method (default method for app-portage/layman-2.3.0 or later)
Create /etc/portage/repos.conf/ directory, if it does not exist yet:
root #mkdir /etc/portage/repos.conf
If you have app-portage/layman-2.3.0 or later installed, you can force a rebuild of layman's repos.conf files:
root #layman-updater -R
Fonte: Layman - Wiki do Gentoo (e para mais detalhes)