It can be even simpler. I use Apache serving static files out of /var/www/gems, and a cron job to rebuild the index every 5 minutes if anything's changed. Caching gems is just `rsync -avz $GEM_HOME/cache <server>:/var/www/gems/`. Publishing internal gems I've built is just scp.

Geminabox is simple until you want to run with a lot (500+) of gems. And then index-building is horrible - it basically never finishes.

The complexity comes in mirroring the right set of other people's gems, which match the exact combined set of all versions used in all your various projects.

There is not a good "pass-through" gem server that just automatically mirrors what it is asked for from RubyGems. There really, really should be.