A bit late in the day, but I've only just joined the community.
I'm a new user to Overture but I've used GPO for a while. I find it easier to use controller #11 (expression) for dynamics, it makes more sense to me than using the modulation wheel. If you check the GPO manual you'll find you can use modulation or expression or breath control.
However not all the instruments in GPO respond to this controller. Some only respond to note velocity. So you'll need to modify the velocity curve to affect these instrument's dynamics.
If music be the food of love, play on.
Kein leben ohne Bach.