I don't know how useful this is, in the case of gems, but you can see the 30 (I think) most popular projects on github, based on number of forks or watchers:
That's where I got the idea from. I was really excited to see what the most popular projects on github were, but this is just the tip of the iceberg b/c they refuse to show the complete leaderboard breakdown.
http://github.com/popular/forked http://github.com/popular/watched