GPO works fine with my ASIO Soundscape Mixtreme drivers ... almost. No problems in Cubase SX. No problems standalone.
When I load GPO Studio, I can select the individual Kontakt players, load instruments and get sound etc. - but if I click Settings Preferences I get error messages (Portaudio error (-9985): Device unavailable or Portaudio error (-9999): Unanticipated host error). Then no sound anymore. I get the same problem if I try to open a gpo file or select one of the custom gpo files from the file menu.
Close it down and reopen, things are fine again unless I replicate the above menu options.
You need to select 2 different drivers when using Studio and Cubase (I am assuming that there in lies the problem.) Usually I just set Cubase to the Direct X driver for my card and Studio to the Multi-Client version.