To create a basic Rack application, all you need is a file named `config.ru`.

# config.ru

run lambda { |env|
  status = 200
  header = {'Content-Type'=>'text/plain'}
  body = StringIO.new("Hello Rack World!")
  return [status, header, body]
}

You can then add files to your git tree and deploy it on your instance by following these instructions: http://wiki.gandi.net/en/simple/instance/ruby#pushing_your_application_to_your_instance