Only use certain gems in a Gemfile if they are already installed, otherwise ignore them.

Gemfile

# In some development environments, different developers have different needs 
# and prefer to use different development gems. Now since Gemfiles are Just Ruby,
# it's pretty easy to work around the different environments.
####################################################################################
# to use:
# 1) put the following code into your Gemfile
# 2) change the `optional_development_gems` array to your custom list
# 3) install none, any, or all of the gems in that list
# 4) export custom_gems=enabled
# 5) bundle install

if ENV['custom_gems'] == 'enabled' then
  optional_development_gems = %w{wirble autotest autotest-rails autotest-fsevent autotest-growl term-ansicolor}
  optional_development_gems.each do |g|
    gem(g) if Gem::Specification.find_by_name(g
  end
end

Updated for the new version of Gem (1.8.8).