First thank you all for the great Ruby GTK+ support. Seems that latest ruby-gdk3-2.2.0 needs currently not existing ruby-atk-2.2.0 emerge: there are no ebuilds to satisfy ">=dev-ruby/ruby-atk-2.2.0[ruby_targets_ruby20]". (dependency required by "dev-ruby/ruby-gdk3-2.2.0[ruby_targets_ruby20,-test]" [ebuild])
I believe this was just fixed - ruby-atk-2.2.0 was added to portage a few hours ago. Please resync.