gemlist_parser.rb

input = ARGF.read

lines = input.split("\n").select { |line|
  !line.empty? && !line.include?("***")
}.map { |line|
  {
    gemname: line.split(" ").first,
    version: line.split(" ").last.gsub(/[(|)]/, ""),
  }
}

puts %{ source "https://rubygems.org" \n\n}

lines.each do |line|
  begin
    puts %{gem "#{line[:gemname]}" }
  rescue Errno::EPIPE
    exit(74)
  end
end

gem list --no-details -l | ruby gemlist_parser.rb > Gemfile

I'm happily surprised to be told that rvm has this feature.

https://twitter.com/rvm_io/status/423344618508673024
https://twitter.com/rvm_io/status/423349650847170560