I was looking for a RubyGems proxy a couple of weeks ago but was unable to find anything suitable. What I would like to find is something similar to Artifactory for Maven. You include the proxy in your Gemfile and if it doesn't have the Gem it downloads it from RubyGems and caches it locally.
This type of proxy wouldn't help in this particular case but it would allow you to keep traffic to RubyGems.org down and also give you the ability to easily host private Gems.
You're probably aware, but it's possible to host private gems in a simple static webserver. We have our CI server copy gems over and run a script that calls "gem generate_index -d /path/to/gems".
Completely featureless, of course. Lacks the newer rubygems.org api (so bundler is stuck downloading the whole index), and obsolete versions will stick around without manual intervention (which, if you have too many, is a pathological case for bundler's resolver).
This type of proxy wouldn't help in this particular case but it would allow you to keep traffic to RubyGems.org down and also give you the ability to easily host private Gems.