require 'rubygems'

gem = Gem::Dependency.new('rails', '=2.1.2')
matches = Gem.source_index.find_name(gem.name, gem.version_requirements)
matches.last.runtime_dependencies.each do |dep_gem|
  puts "#{dep_gem.name}, #{dep_gem.version_requirements}" # => nil, nil, nil, nil, nil, nil
end
# >> rake, >= 0.8.1
# >> activesupport, = 2.1.2
# >> activerecord, = 2.1.2
# >> actionpack, = 2.1.2
# >> actionmailer, = 2.1.2
# >> activeresource, = 2.1.2