I ran across this group today:

http://vis.versilstudios.net/index.html

They have some nice sounding freebies, including a very delicate sounding upright, a zither, some decent sounding orchestral bits, and heaps of percussion craziness.

http://vis.versilstudios.net/free.html

I think some of these have the potential to mix well with GPO and IO.