Check for uninstalled gem dependencies.

gem-depcheck

#!/usr/bin/ruby
require 'rubygems'
installed_gems = Gem::Dependency.new '', Gem::Requirement.default
missing = []
# mind the short-hand, looping over installed gem specs
specs = Gem.source_index.search installed_gems
specs.each do |spec|
  gem = Gem::Dependency.new spec.name, spec.version
  # looping over gem dependencies
  specs = Gem.source_index.search gem
  deps = specs.first.runtime_dependencies
  deps.each do |dep|
    # check if the dependency is missing
    specs = Gem.source_index.search dep
    if specs === [] then
      missing.push dep
    end
  end
end
print missing.sort.uniq.collect{|spec| "#{spec}\n"}