Is there a gem that you want to include sometimes and not others? For instance, do you have multiple versions of staging with slightly different gemsets? You can manage that with custom groups.
group :apple do gem 'some_gem' end group :orange do gem 'some_other_gem' end
RAILS_GROUPS=apple,orange rails console > Rails.groups [:default, "development", "apple", "orange"]
Rails.groups is passed to