I've updated my Gemfile syntax file, adding a dash of color and making sure it supports all elements of the Bundler DSL. You can get it here. You'll also need to tell vim to automatically use it when opening a Gemfile by adding this file.

This is how it looks with the ir_black colorscheme:

source :rubygems
gem 'rails'
gem 'hoptoad_notifier'
gem 'newrelic_rpm', :require => false
gem 'mysql2'
gem 'devise'

# ... etc ...

platforms :ruby_18 do
  gem 'system_timer'
  gem 'fastercsv'
end

group :development, :test do
  gem 'rspec-rails', '2.0.0.beta.20'
end

Enjoy!