This could serve as template for Gemfiles in all DM gems (that one in particular is from dm-core)

dm_gemfile_template.rb

# If you're working on more than one datamapper gem at a time, then it's
# recommended to create a local Gemfile and use this instead of the git
# sources. This will make sure that you are  developing against your
# other local datamapper sources that you currently work on. Gemfile.local
# will behave identically to the standard Gemfile apart from the fact that
# it fetches the gems from local paths. This means that you can use the
# same environment variables, like ADAPTER when running bundle commands.
# Gemfile.local is added to .gitignore, so you don't need to worry about
# accidentally checking local development paths into git.
#
#   bundle exec rake local_gemfile
#
# will give you a Gemfile.local file that points to your local clones of
# the various datamapper gems. It's assumed that all datamapper repo clones
# reside in the same directory. You can use the Gemfile.local like so for
# running any bundle command:
#
#   BUNDLE_GEMFILE=Gemfile.local bundle foo
#
# Specify which adapter(s) should be part of the bundle by setting an ENV var.
# This also works when using the Gemfile.local
#
#   bundle foo                        # dm-sqlite-adapter
#   ADAPTER=mysql bundle foo          # dm-mysql-adapter
#   ADAPTERS=sqlite,mysql bundle foo  # dm-sqlite-adapter and dm-mysql-adapter
#
# You can also use the ADAPTER(S) variable when using the Gemfile.local and
# running specs against selected adapters.
#
# For easily working with adapters supported on your machine, it's recommended
# that you first install all adapters that you are planning to use / work on
# by doing something like
#
#   ADAPTERS=sqlite,mysql,postgres bundle install
#
# This will clone the various repositories and make them available to bundler.
# Once you have them installed you can easily switch between adapters for the
# various development tasks. Running something like
#
#   ADAPTERS=mysql bundle exec rake spec
#
# will make sure that the dm-mysql-adapter is part of the bundle, and will be used
# when running the specs.
#
# Specify which plugin(s) should be part of the bundle by setting an ENV var.
# This also works when using the Gemfile.local
#
#   bundle foo                                 # dm-migrations
#   PLUGINS=dm-validations bundle foo          # dm-migrations and dm-validations
#   PLUGINS=dm-validations,dm-types bundle foo # dm-migrations, dm-validations and dm-types
#
# Of course you can combine the PLUGINS env var with the ADAPTERS env var to test out
# certain combinations of adapter/plugin combinations.
#
# To speed up running specs and other tasks, it's recommended to run
#
#   bundle lock
#
# after running 'bundle install' for the first time. This will make 'bundle exec' run
# a lot faster compared to the unlocked version. With an unlocked bundle you would
# typically just run 'bundle install' from time to time to fetch the latest sources from
# upstream. When you locked your bundle, you need to run
#
#   bundle install --relock
#
# to make sure to fetch the latest updates and then lock the bundle again. Gemfile.lock
# is added to the .gitignore file, so you don't need to worry about accidentally checking
# it into version control.

source 'http://rubygems.org'

DATAMAPPER = 'git://github.com/datamapper'


group :runtime do # Runtime dependencies (as in the gemspec)

  if ENV['EXTLIB']
    gem 'extlib',        '~> 0.9.15',      :git => "#{DATAMAPPER}/extlib.git"
  else
    gem 'activesupport', '~> 3.0.0.beta3', :git => 'git://github.com/rails/rails.git', :require => nil
  end

  gem 'addressable',     '~> 2.1'

end

group(:development) do # Development dependencies (as in the gemspec)

  gem 'rake',           '~> 0.8.7'
  gem 'rspec',          '~> 1.3'
  gem 'yard',           '~> 0.5'
  gem 'rcov',           '~> 0.9.7'
  gem 'jeweler',        '~> 1.4'

end

group :quality do # These gems contain rake tasks that check the quality of the source code

  gem 'yardstick',      '~> 0.1'
  gem 'metric_fu',      '~> 1.3'
  gem 'reek',           '~> 1.2.7'
  gem 'roodi',          '~> 2.1'

end

group :datamapper do # We need this because we want to pin these dependencies to their git master sources

  DM_VERSION = '~> 0.10.3'

  gem 'dm-core', DM_VERSION, :path => File.dirname(__FILE__) # Make ourself available to the adapters

  adapters = ENV['ADAPTER'] || ENV['ADAPTERS']
  adapters = adapters.to_s.gsub(',',' ').split(' ') - ['in_memory']

  unless adapters.empty?

    DO_VERSION     = '~> 0.10.2'
    DM_DO_ADAPTERS = %w[sqlite postgres mysql oracle sqlserver]

    gem 'data_objects',  DO_VERSION, :git => "#{DATAMAPPER}/do.git"

    adapters.each do |adapter|
      if DM_DO_ADAPTERS.any? { |dm_do_adapter| dm_do_adapter =~ /#{adapter}/  }
        adapter = 'sqlite3' if adapter == 'sqlite'
        gem "do_#{adapter}", DO_VERSION, :git => "#{DATAMAPPER}/do.git"
      end
    end

    gem 'dm-do-adapter', DM_VERSION, :git => "#{DATAMAPPER}/dm-do-adapter.git"

    adapters.each do |adapter|
      gem "dm-#{adapter}-adapter", DM_VERSION, :git => "#{DATAMAPPER}/dm-#{adapter}-adapter.git"
    end

  end

  plugins = ENV['PLUGINS'] || ENV['PLUGIN']
  plugins = (plugins.to_s.gsub(',',' ').split(' ') + ['dm-migrations']).uniq

  plugins.each do |plugin|
    gem plugin, DM_VERSION, :git => "#{DATAMAPPER}/#{plugin}.git"
  end

end

dm_local_gemfile_task.rb

desc "Support bundling from local source code (allows BUNDLE_GEMFILE=Gemfile.local bundle foo)"
task :local_gemfile do |t|

  root              = Pathname(__FILE__).dirname.parent
  datamapper        = root.parent

  source_regex     = /DATAMAPPER = 'git:\/\/github.com\/datamapper'/
  gem_source_regex = /:git => \"#\{DATAMAPPER\}\/(.+?)(?:\.git)?\"/

  root.join('Gemfile.local').open('w') do |f|
    root.join('Gemfile').open.each do |line|
      line.sub!(source_regex,     "DATAMAPPER = '#{datamapper}'")
      line.sub!(gem_source_regex, ':path => "#{DATAMAPPER}/\1"')
      f.puts line
    end
  end

end

dm_rakefile_change.rb

## Rakefile (OPTIONAL - needs discussion)

# Doing this would of course force bundler on everyone wanting to run the rake tasks
# This needs more discussion but it would definitely dry up dependency declaration.

require 'rubygems'
require 'bundler'
#..
  Jeweler::Tasks.new do |gem|
    #...
    # This will add everything in the :runtime group as a runtime,
    # and everything in the :development group, as a development dependency
    gem.add_bundler_dependencies(:runtime, :development)
    #...
  end
#..