For some reason, GPO uses mod wheel to control volume and on their website
 
They say to control volume,
"It is always best to record Mod Wheel data at the beginning of every MIDI sequence track in order to start with the correct initial volume."
 
But where do you record Mod wheel data? :( I tried in prv and added Wheel and tried changing but it doesn't change mod wheel in GPO...
 
Can somebody help? I'd really appreciate it.

All you need to do is enter a starting volume event in the event list. You may need to first enter a note, but you can change it from a note to controller event. then just set a starting volume, for example 80. you can use controller number 1, 7 or 11. This will ensure that you will always be able to hear the instruments on GPO.