Run it as `$ ruby gemfile_parser.rb > Gemfile ` to generate a new Gemfile from a Gemfile.lock

gemfile_parser.rb

require 'bundler'

lockfile = Bundler::LockfileParser.new(Bundler.read_file('Gemfile.lock'))

specs = lockfile.specs
gems_hash = Hash.new.tap do |h|
  specs.each do |s|
    h[s.name] = {
      spec: s,
      dependencies: s.dependencies.map(&:name)
    }
  end
end

dependencies = gems_hash.keys && gems_hash.values.map { |h| h[:dependencies] }.flatten.uniq.sort
dependencies.each { |dep| gems_hash.delete(dep) }

relevant_specs = gems_hash.values.map { |h| h[:spec] }

puts "source 'https://rubygems.org'"
puts

relevant_specs.each do |s|
  if s.source.to_s =~ /https:\/\/rubygems.org/
    puts "gem '#{s.name}', '#{s.version}'"
  elsif s.source.is_a?(Bundler::Source::Git)
    uri = s.source.uri
    branch = s.source.branch
    ref = s.source.ref
    puts
    puts "git '#{uri}', branch: '#{branch}', ref: :#{ref} do"
    puts "  gem '#{s.name}'"
    puts "end"
    puts
  end
end
puts