this is a rough draft, and may need revision.
Rubygems is a package management system to extend the ruby language.
Gems are packaged wonky in portage so this page will explain how to work with rubygems rather than fight it. Arch wiki outlines a similar method.
Emerge ruby & rubygems:
root #emerge ruby rubygems
Remove bunk gems:
root #emerge -c dev-ruby/rake dev-ruby/racc dev-ruby/json dev-ruby/rdoc
Flush bunk gems bad specs:
root #mv /usr/local/lib64/ruby/gems/2.1.0/specifications/ /usr/local/lib64/ruby/gems/2.1.0/specifications.backup
Fetch gems via rubygems:
root #gem install rdoc rubygems-update
update gem system:
root #update_rubygems root #gem update --system root #gem pristine --all
Now you're ready to use gem to manage gems. The reason this came about is that bundler can't be run with out conflicts.