This is something to be proud of, and github is great work.
It's not really accurate, though because this lists every user's fork of a gem as a distinct gem. Here's a script I put together to get a different count, which I believe is more of a "real" count.
Forks aren't gem-enabled by default. Meaning, the forker had to go out of their way to indicate there is something different about their gem. Is it possible there isn't unique code on some of these gems? Sure, but I wouldn't discount them outright.
It's not really accurate, though because this lists every user's fork of a gem as a distinct gem. Here's a script I put together to get a different count, which I believe is more of a "real" count.
http://gist.github.com/120207
It's 3140 as I write this, not 4766.