Well, it uses modern C++ idioms that didn't exist whet Gaigen 2 was created (e.g. C++11). Mostly, these eliminate the performance-related need for a code generator (and incidentally, serves as great example of how much further C++11 moved the language in practical terms).
But more importantly to me, I think versor is a better-designed library/API and it implements more of the things that are needed to effectively use geometric algebra in practice. Most of these "additional things" are documented in Articulating Space, which you've already linked to. If you use versor, you can make direct use of a working, tested implementation (instead of trying to rewrite them to work with Gaigen 2's API).
Versor is also actively maintained, which I consider a feature.
That said…Gaigen 2 is perfectly serviceable and performant, so if it meets your needs, I don't think you should feel like you've made a bad choice. It works, it's fast, and as you pointed out, it generates output for languages other than C++. Prior to discovering versor, that's what I used, too.