Tetters, the prototype of my GP38-2 has a 645-16 engine and your GP9s have 567-16 engines (not turbo charged), so that you’ll need a different decoder. Have a look at the ESU page to find the item-# for the right LokSound 3.5 decoder (
http://www.esu.eu/index.php?id=339 ).
edit..... #72428 is not the one I use, but it might be the right decoder for your geep. Are you sure about the air horn?
BTW, no one offers the right decoder for a GP38-2. The 645-16 engine of the GP38 is not turbo charged and all the decoders on offer have the turbo sound. I listened to a lot of videos on Youtube and decided to go for the normally aspirated 645-12 engine, because the sound of the 12 cylinder comes much closer than the sound of a 16 cylinder turbo charged engine.