Took a quick look, GS Collection seems to be a similar library like Google Guava; a nice functional library helper to perform operation against Java collection.
The examples they give seem to be pretty much standard functionality of the Guava Collections2 class. With Guava quickly becoming one of those standard libraries that just gets automatically added to virtually every new project, I don't see much reason to use this.
GS Collections also seems to have new implementations of the basic Java collections (implementations of List<T>, Set<T>, etc.). I haven't studied them sufficiently to say whether I like them better, though.
At first glance I think I dig Guava's API a little more, but I haven't looked closely enough to really have an opinion.
Not to take away from the act of open sourcing it, but I think Google's guava library is a lot more polished. But I expect Google has spent a heck of a lot more manhours on their library.