Erlang is available via emerge, but may need to be unmasked. Install it with these commands:
sudo echo 'dev-lang/erlang ~amd64' >> /etc/portage/package.accept_keywords
sudo echo 'dev-util/rebar ~amd64' >> /etc/portage/package.accept_keywords
emerge --ask dev-lang/erlang
emerge --ask dev-util/rebar