Birthday! Waiting for my patch though:

rubygems/rubygems#2061

.gitmodules

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+[submodule "task"]
+	path = task
+	url = git://github.com/godfat/gemgem.git

README.md

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+# gem-compact [![Build Status](https://secure.travis-ci.org/godfat/gem-compact.png?branch=master)](http://travis-ci.org/godfat/gem-compact)
+
+by Lin Jen-Shin ([godfat](http://godfat.org))
+
+## LINKS:
+
+* [github](https://github.com/godfat/gem-compact)
+* [rubygems](https://rubygems.org/gems/gem-compact)
+
+## DESCRIPTION:
+
+Clean up gems for all the paths, including development dependencies.
+
+## REQUIREMENTS:
+
+* Tested with MRI (official CRuby), Rubinius and JRuby.
+
+## INSTALLATION:
+
+    gem install gem-compact
+
+## SYNOPSIS:
+
+    gem compact
+    gem compact rake
+
+## CONTRIBUTORS:
+
+* Lin Jen-Shin (@godfat)
+
+## LICENSE:
+
+Apache License 2.0 (Apache-2)
+
+Copyright (c) 2017, Lin Jen-Shin (godfat)
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+<http://www.apache.org/licenses/LICENSE-2.0>
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

Rakefile

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+
+begin
+  require "#{__dir__}/task/gemgem"
+rescue LoadError
+  sh 'git submodule update --init --recursive'
+  exec Gem.ruby, '-S', $PROGRAM_NAME, *ARGV
+end
+
+Gemgem.init(__dir__) do |s|
+  s.name    = 'gem-compact'
+  s.version = '0.5.0'
+end

gem-compact.gemspec

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+# -*- encoding: utf-8 -*-
+# stub: gem-compact 0.5.0 ruby lib
+
+Gem::Specification.new do |s|
+  s.name = "gem-compact".freeze
+  s.version = "0.5.0"
+
+  s.required_rubygems_version = Gem::Requirement.new(">= 0".freeze) if s.respond_to? :required_rubygems_version=
+  s.require_paths = ["lib".freeze]
+  s.authors = ["Lin Jen-Shin (godfat)".freeze]
+  s.date = "2017-11-04"
+  s.description = "Clean up gems for all the paths, including development dependencies.".freeze
+  s.email = ["godfat (XD) godfat.org".freeze]
+  s.files = [
+  ".gitmodules".freeze,
+  "README.md".freeze,
+  "Rakefile".freeze,
+  "gem-compact.gemspec".freeze,
+  "lib/rubygems/commands/compact_command.rb".freeze,
+  "lib/rubygems_plugin.rb".freeze,
+  "pkg/gem-compact-0.5.0.gem".freeze,
+  "task/README.md".freeze,
+  "task/gemgem.rb".freeze]
+  s.homepage = "https://github.com/godfat/gem-compact".freeze
+  s.licenses = ["Apache-2".freeze]
+  s.rubygems_version = "2.7.1".freeze
+  s.summary = "Clean up gems for all the paths, including development dependencies.".freeze
+end

lib/rubygems/commands/compact_command.rb

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+
+class Gem::Commands::CompactCommand < Gem::Command
+  def description
+    'Clean up gems for all the paths'
+  end
+
+  def initialize
+    super('compact', description)
+  end
+
+  def execute
+    require 'rubygems/commands/cleanup_command'
+
+    cmd = Gem::Commands::CleanupCommand.new
+    args, build_args = options.values_at(:args, :build_args)
+    args.unshift('--no-check-development')
+
+    with_path do
+      cmd.invoke_with_build_args(args, build_args)
+    end
+  end
+
+  private
+
+  def with_path
+    path = Gem.path
+    path.each do |dir|
+      Gem.use_paths(dir)
+      yield
+    end
+  ensure
+    Gem.use_paths(*path)
+  end
+end

lib/rubygems_plugin.rb

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+
+require 'rubygems/command_manager'
+Gem::CommandManager.instance.register_command(:compact)