I have created a template in Overture 3 to use with GPO. It includes Percussion instruments in GPO Player 5, with Timpani in Slot 1 and Cymbals in Slot 4. I have the Timpani track set to Slot 1, where the Timpani is, and the Percussion track set to Slot 4, where the Cymbals are.
The problem is that I hear no Cymbals playing on the Percussion track in Overture. I can't live without my Cymbals! Can't compose without 'em either.
The percusion staves in Overture have a restricted note range. So the notes in it may not fit the real notes you need to play in GPO. One way to fix it can be using normal staves (this is, G key or F key). Anyway I think there's an article in tutuorials on Overture and GPO discussing this issue.