AFAIK, Mixcraft dont handle Multiple-outs yet. That means that YOU CAN use GPO without problem, BUT all the instruments will sound trough the same stereo channel. So, if you want to assign a Violin to a different track to apply and effect different than the rest of the instruments, well, that´s not possible.
This is not a GPO issue, most of the orchestral libraryes, Kontakt based or not, works this way. Also percussion kits do the same.
So, if you are user of Mixcraft, request your developers that feature. Is usefull, and most of others programs has this feature already. Said that, maybe you dont need it, and working trough a single channel is enough. Lot of people work that way.