With this App you can play guitar on your midi compatible piano.
Use the Apple Camera Connection Kit or a compatible device zu connect your piano to the iPad. Now you can use the Keys marked in the picture below to play your piano like a guitar. Or you can use the touch strings in the app.
I recommend you to use a separate audio sample app with real guitar audio samples. Configure the other app to listen, on midi port 2, the standard output port of this app or change the settings.