After a hiatus of more than 2 years Rubinius is back with Gentoo. Rubinius 1.2.4 got added this weeked as dev-lang/rubinius, making it the 5th ruby provider natively supported in Gentoo. If you want to install your ruby packages (also) for Rubinius you should add "rbx" to RUBY_TARGETS in /etc/make.conf. That will ensure that all packages that have been marked as ready for rubinius will be installed for it.
Right now that list is quite small still, but this should improve over time. If you would like to see a package marked for Rubinius (or another missing ruby implementation), then please open a bug for it. Please add the output of a successful build with both FEATURES=test and USE=doc to verify that everything works as expected.
Thanks to the people participating in bug 334177 for testing, initial ebuilds, and support.