My latest project's Gemfile, implements bundle group pattern

application.bundler.rb

# Load this file by replacing the if defined?(Bundler) block in config/application.rb with:
#   require File.expand_path('../application.bundler', __FILE__)

if defined?(Bundler)
  Class.new Rails::Railtie do
    console do |app|
      Bundler.require(:console)
      require 'irbtools/configure'
      Irbtools.add_package :more # adds this extension package
      # here you can edit which libraries get loaded. See the irbtools README for details.
      Irbtools.start
    end
  end
  groups = {
    # If you precompile assets before deploying to production, use this line
    assets:     %w(development test),
    monitoring: %w(staging production),
    security_analysis: %w(development test),
  }
  # List the groups that are always loaded, followed by the ones that are selectively loaded based on environment
  Bundler.require(:rails3, :web_server, :data_store, :async_jobs, :templates, :javascript, :users, :metrics, :messaging, :email, :enhancers, :mobile, *Rails.groups(groups))
  #Bundler.require(*Rails.groups(:assets => %w(development test)))
  # If you want your assets lazily compiled in production, use this line
  # Bundler.require(:default, :assets, Rails.env)
end

Gemfile

##### See Gemfile.md for detailed notes on specific gems #####
source 'https://rubygems.org'
# Follows the bundler group pattern described here:
#   http://iain.nl/getting-the-most-out-of-bundler-groups

# Gemfile.mine in root dir allows locally custom gems.
# NOTE: Doing this will change the Gemfile.lock - commit with care.
eval File.read(File.join(File.dirname(__FILE__), 'Gemfile.mine')) if File.exists? File.join(File.dirname(__FILE__), 'Gemfile.mine')

ruby '1.9.3'

#################### Rails 3.2 ####################
# NOTE on branching:  We are using the rails 4 track gems as much as possible,
#                     because in most cases they retain support for Rails 3,
#                     and there will fewer issues with upgrading to Rails 4 when the time comes.
#                     Where we need actual different gems (e.g. rails proper) we have a rails4 group (see EOF)
group :rails3 do
  gem 'rails', '~> 3.2.11'
  # Not compatible with Rails4, see :rails4 group for alternative
  gem 'simple_form', '~> 2.0.4'
  # Backports default Rails4 functionality to Rails3
  gem 'dismissible_helpers', '~> 0.1.5'
end
group :assets do
  gem 'sprockets-rails', '~> 0.0.1'
  gem 'sass-rails', '~> 3.2.5'
  gem 'coffee-rails', '~> 3.2.2'
  gem 'turbo-sprockets-rails3', '~> 0.3.6'
end
#################### /Rails 3.2 ####################

group :web_server do
  # The Webserver
  gem 'puma', '~> 2.0.0.b4' # 1.6.3 has issues with NewRelic
  # Allows rack to rewrite requests, no need for Apache mod_rewrite
  gem 'rack-rewrite', '~> 1.3.3'
end

group :data_store do
  # The Database
  gem 'pg', '~> 0.14.1'
  # More power to the Database
  gem 'pg_power', '~> 1.3.0' #, github: 'acquaintable/pg_power', branch: 'rails4'
  # Allows PostgreSQLto use the new HSTORE from Rails 4
  gem 'activerecord-postgres-hstore', '0.5.2'
  # Bit fields for Active Record - Follows conventions of standard boolean columns.
  gem 'flag_shih_tzu', '~> 0.3.2'
  # Neo4j REST API
  gem 'neography', '~> 1.0.5'
  # Replicates wholly, or partially, ActiveRecord (PG) models into Neo4j for faster querying
  # Manages synchronization of updates to records across both data stores
  gem 'neoid', '~> 0.1.2'
  # Versioning Things and Soft-Deletes
  gem 'vestal_versions', github: 'justintanner/vestal_versions'
  # Voting On Things
  gem 'thumbs_up', '~> 0.6.2'
  # Map any methods on any Ruby objects to a CSV export, and vice verse.
  gem 'csv_pirate', '~> 5.0.7'
end

group :async_jobs do
  # The Cache - Redis can back Object Caches, I18n, Session, HTTP Cache
  gem 'redis', '~> 3.0.2'
  gem 'redis-namespace', '~> 1.2.1'
  gem 'redis-rails', '~> 3.2.3'
  gem 'redis-store', '~> 1.1.3'

  # Our Queue
  gem 'sidekiq', '~> 2.7.0'
  # Add failure tracking and failure tab to sidekiq UI
  gem 'sidekiq-failures', '~> 0.1.0'
  # Queue Manager (Hires and Fires workers)
  gem 'autoscaler', '~> 0.2.0'
end

group :templates do
  # View Template Language
  gem 'slim', '~> 1.3.5'
  gem 'slim-rails', '~> 1.1.0'
  # Views can be and should be 100% DRY
  gem 'dry_views', '~> 0.0.2'
  # Markdown Parser (for copy)
  gem 'redcarpet', '~> 2.2.2'
  # To avoid indecent exposure of instance variables in the view
  gem 'decent_exposure', '~> 2.0.1'
  gem 'cells', '~> 3.8.6'#, :path => '/Users/pboling/Documents/src/other/cells' #:git => 'git://github.com/acquaintable/cells.git'
end

# TODO: Determine which of these can be moved into :assets group
group :javascript do
  # Base for JSON
  gem 'json', '~> 1.7.6'
  # Fast JSON Object parsing and marshalling
  gem 'oj', '~> 2.0.3'
  # Implant jQuery directly into the Asset Pipeline ========>
  gem 'jquery-rails', '~> 2.1.4' # '~> 2.2.0' updates to jQuery 1.9, which is not yet compatible with foundation, see:
  # https://github.com/zurb/foundation/pull/1432
  # Enable the AMD design pattern for Javascript modules and integrate into the Asset Pipeline ========>
  gem 'requirejs-rails', github: 'jwhitley/requirejs-rails'
end

group :users do
  # Registration, Authentication, Invitation, Confirmation
  gem 'devise', github: 'idl3/devise', branch: 'rails4'
  gem 'devise_invitable', github: 'sunny36/devise_invitable', :ref => '5af50a925e0a41cc648b124ffe7de0f8f081481e'
  gem 'devise-async', '~> 0.5.1'
  # Facebook Oauth
  gem 'koala', '~> 1.6.0'
  gem 'omniauth', '~> 1.1.1'
  gem 'omniauth-facebook', '~> 1.4.1'
  # Authorization
  gem 'cancan', '>= 1.6.8'
  gem 'rolify', '>= 3.2.0'
  # Stateful Behavior
  gem 'state_machine', '~> 1.1.2'
end

group :metrics do
  # User Metrics
  gem 'km', '~> 1.1.3'
  # Send errors to airbrake.io
  gem 'airbrake', '~> 3.1.7'
  # Report missing i18n translations
  gem 'i18n-airbrake', github: 'acquaintable/i18n-airbrake', branch: 'configuration_options'
  # Code Metrics
  gem 'newrelic_rpm', '~> 3.5.5.540.dev'
  # The rails app administration tool
  gem 'rails_admin', '~> 0.4.3'
  # Debugging toolbar, heir of rack-bug
  #gem 'rack-insight'
end

group :messaging do
  # Add messages to your flash messaging stack whenever you need to, array-style
  gem 'stackable_flash'
  # Use the cookie for transporting the flash messages and javascript to render them into the view
  gem 'cacheable_flash'
end

group :email do
  # Third Party Email Service
  gem 'sendgrid', '>= 1.0.1'
  # Redirect all email in non-Production environments to alternate, safe, addresses
  gem 'sanitize_email', '~> 1.0.5'
end

group :enhancers do
  # Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
  gem 'turbolinks', '~> 0.6.1'
  # Mini Controllers & tiny mounted Rack apps inside Rails for widgety things
  # XML Parsing
  gem 'nokogiri', '~> 1.5.6'
  # Yet another framework
  # TODO: Document why this is here.  What relies on this?
  gem 'sinatra', :require => nil
  # HTML5 Boilerplate
  gem 'html5-rails'
  # Possessive form of a string
  gem 'possessive'
  # Convert numbers to words 1 => 'one'
  gem 'numbers_and_words'
end

group :mobile do
  # Mobile Push
  gem 'urbanairship', '~> 2.2.0'
end

group :security_analysis do
  # Check for Security Holes
  gem 'brakeman'
end

# Gems used only for assets and not required
# in production environments by default.
group :assets do
  # AWS RUBY APIs
  # NOTE: This gem is almost threadsafe
  # See: https://github.com/mperham/sidekiq/wiki/Problems-and-Troubleshooting
  gem 'aws-sdk', '~> 1.8.0'
  # Assets stored on AWS S3
  # Goal is to send compiled assets to AWS when FOGGY is not set, and when it is set to 'true'
  gem 'asset_sync', '~> 0.5.4', {}.merge(!ENV['FOGGY'].nil? && ENV['FOGGY'] != 'true' ? {require: false} : {})
  gem 'uglifier', '>= 1.3.0'
  gem 'handlebars_assets', '~> 0.8.1'
  gem 'compass-rails', github: 'ai/compass-rails', branch: 'rails4' #'>= 1.0.3'
  gem 'compass-h5bp'
  gem 'zurb-foundation', '>= 3.2.3'
  gem 'foundation-icons-sass-rails', '~> 2.0.0'
end

group :development do
  # Find bad queries, TODO: evaluate: uniform_notifier for use with bullet
  gem 'bullet', '~> 4.3.0'
  gem 'quiet_assets', github: 'evrone/quiet_assets'
  # Improve the development error experience
  gem 'better_errors', '>= 0.3.2'
  gem 'binding_of_caller'
  # Annotate models, fixtures, and model specs
  gem 'annotate', '>=2.5.0'

  # Allows us to magically convert directories of haml templates to slim templates
  gem 'haml2slim', '>= 0.4.6'
  gem 'haml', '>= 3.1.7'
  gem 'haml-rails', '>= 0.3.5'
end

group :development, :test do
  gem 'powder', '~> 0.1.8'
  gem 'rails_best_practices', '~> 1.13.2'
  #gem 'ruby-prof', '~> 0.11.3', require: false
  gem 'rspec-rails', '>= 2.12'
  gem 'fabrication', '>= 2.5.2'
  gem 'parallel_tests', '~> 0.9.2'
  gem 'teabag', github: 'acquaintable/teabag'
  gem 'pry', '~> 0.9.10'
  gem 'pry-rails'
  gem 'pry-doc'
  gem 'pry-remote'
  # Guard
  gem 'guard'
  gem 'guard-spork'
  gem 'guard-rspec', '~> 2.3.3'
  gem 'rb-inotify', :require => false
  gem 'rb-fsevent', :require => false
  gem 'wdm', :platforms => [:mswin, :mingw], :require => false
  # Mac OS X Notification center
  gem 'terminal-notifier-guard'
  # Spork
  gem 'spork-rails'
end

group :test do
  gem 'remarkable_activerecord', '>=4.0.0.alpha2'
  gem 'cane', '~> 2.5.0'
  gem 'test-unit', '~> 2.5.3'
  gem 'mocha', '0.13.1', require: false
  #gem 'rspec-apotomo', :git => 'git://github.com/kristianmandrup/rspec-apotomo.git', :ref => '03a2ac2f526fa668ee3a4bdb09cbc41693931ca3'
  gem 'rspec-cells', github: 'acquaintable/rspec-cells'
  gem 'shoulda-matchers', github: 'acquaintable/shoulda-matchers'
  gem 'bourne', github: 'thoughtbot/bourne'
  gem 'capybara', '>= 2.0.2'
  gem 'capybara-email'
  gem 'poltergeist', github: 'brutuscat/poltergeist'
  gem 'vcr', '~> 2.3.0'
  gem 'fakeweb', '~> 1.3.0'
  #gem 'autotest', '4.4.6'
  #gem 'autotest-rails', '4.1.2'
  gem 'simplecov', '~> 0.7.1', require: false
  gem 'database_cleaner', '>= 0.9.1'
  gem 'email_spec', '>= 1.4.0'
end

# These gems are only loaded when rails is loaded in console mode, see condition near top of config/application.rb
group :console do
  gem 'irbtools', :require => false, github: 'acquaintable/irbtools'
  gem 'irbtools-more', :require => false
  gem 'terminal-notifier'
end

#################### Gems Under Evaluation ####################
# This group is never loaded by default in any environment
# We are in process of evaluating them, and this ensures they are bundle-able,
#   for at least a nominal level of integration awareness
group :evaluation do
  # Abstraction Layer over the WebSocket protocol
  gem 'websocket', '~> 1.0.6'
  # Geolocations
  gem 'geokit', '~> 1.6.5'
  gem 'geokit-rails3', github: 'acquaintable/geokit-rails3'
  #gem 'airbrake-statsd', '~> 0.2.1'
  # App, Service & Environment Config
  gem 'figaro', github: 'acquaintable/figaro', branch: 'rails4'
  # User Metrics
  gem 'analytical', '~> 3.0.12'
  # But User Metrics from the client-side!
  gem 'event_tracker', github: 'acquaintable/event_tracker'
    # AJAX file upload
  gem 'remotipart'
  # Automated Sitemap and SEO
  gem 'sitemap_generator'
  # A/B Testing
  gem 'split', '~> 0.4.6'
  # Mobile Response
  gem 'mobylette', '~> 3.3.2'
  # Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
  gem 'jbuilder', '~> 0.9.1'
  # To use dummy images built on-demand for rapid prototyping
  # Depends on an install of imagemagick and the rmagick gem
  # TODO:Verify Setup instructions for ImageMagick before uncommenting rmagick
  #gem 'rmagick'
  #gem 'dummy_image', :git => 'git://github.com/davelyon/dummy_image.git'
end
####################/ Gems Under Evaluation ####################

#################### Rails 4.0 ####################
#group :rails4 do
#  gem 'rails',     github: 'rails/rails'
#  gem 'journey',   github: 'rails/journey'
#  gem 'arel',      github: 'rails/arel'
#  gem 'simple_form', github: 'plataformatec/simple_form', branch: 'rails_4'
#  gem 'activerecord-session_store', github: 'rails/activerecord-session_store'
#  TODO: These are deprecated functionality of Rails 3 removed from Rails4
#         We will use this to transition, but ultimately change our code to not rely on it
#  gem 'activerecord-deprecated_finders', github: 'rails/activerecord-deprecated_finders'
#  gem 'protected_attributes', github: 'rails/protected_attributes'
#end
#group :assets do
#  gem 'sprockets-rails', github: 'rails/sprockets-rails'
#  gem 'sass-rails',   github: 'rails/sass-rails'
#  gem 'coffee-rails', github: 'rails/coffee-rails'
#end
#################### /Rails 4.0 ####################