You can run custom ruby gems from any path, for instance local machine, LAN or remotely, without the necessity to go over the official ruby gem index.
Specifying a :path attribute will install the gem from that path on your local machine.
gem "foreman", :path => "/Users/pje/my_foreman_fork"
Alternately, specifying a :git attribute will install the gem from a remote git repository.
gem "foreman", :git => "git://github.com/pje/foreman.git"
# ...or at a specific SHA-1 ref
gem "foreman", :git => "git://github.com/pje/foreman.git", :ref => "bf648a070c"
# ...or branch
gem "foreman", :git => "git://github.com/pje/foreman.git", :branch => "jruby"
# ...or tag
gem "foreman", :git => "git://github.com/pje/foreman.git", :tag => "v0.45.0"
Also, for some firewalls the git protocol may be an issue. In such cases try using https instead.
gem "foreman", :git => "https://github.com/pje/foreman.git"