Hi Danny,
Using GPO as your MIDI playback device, you set the instruments up in the Aria player. You only have to set the channel numbers in Notation Composer to match the channel number of the appropriate instrument in the Aria Player. (The instrument grid that typically shows up in the Staff Setup is only for GM (general MIDI) sets, which GPO definitely is not
).
So, for example, in setting up the following file that I've opened in Notation Composer, I would want to load a Piccolo in the first slot (with channel 1), a flute in the second slot (with channel 2), an oboe in the third slot (with channel 3), etc.
AriaComposerChannelNumbers.png
Have fun
Sherry