I just bought myself a copy of GPO yesterday and am having one small problem:
Every instrument except the piano has a really low volume level (so much that you cant really hear it).
I can fix this by turning the volume all the way up or by moving the mod wheel (in Kontakt) all the way up, the mod wheel gives me the most volume. The only thing is this doesn't save when I exit the project so when I come to work on it again i have to go through each patch and push the mod wheel up on each instrument.
I'm using GPO as a VSTi in Nuendo 2.