http://www.redmine.org/projects/redmine/wiki/RedmineInstall
Some packages which redmine depends on also need to support ruby-2.0. dev-ruby/i18n:0.6 dev-ruby/jquery-rails dev-ruby/rails:3.2
dev-ruby/jquery-rails dev-ruby/rails:3.2 still miss ruby-2.0 support.
The version available in my overlay supports ruby20 now.
*redmine-2.4.6 (05 Aug 2014) 05 Aug 2014; Manuel Rüger <mrueg@gentoo.org> +redmine-2.4.6.ebuild: NMU. Version bump. See bug #491758. Add ruby20 support. Fixes bug #484484.