http://rubygems.org/gems/make-rubygem-debs

gem install make-rubygem-debs

This is a simple script to generate .debs for all of the gems in a
Bundler-enabled Ruby on Rails webapp, using fpm.  To use it:

$ make-rubygem-debs /path/to/my/rails/app

It will create a 'rubygem-debs' sub-directory and make debs for each rubygem
dependency there for you.  You can then do whatever you need to with them.