TheRealZap is right: On compatible iPhones and iPads, Mobile In works with any iOS app that uses CoreAudio for audio input.  One thing to consider with GK2 is that it's pretty heavy on its own and once you plug a regular 1/4" guitar cable into it, that's a lot of potential stress on the 30-pin dock connector.  Mobile In also connects directly to the dock connector, but we made it much smaller and lighter to minimize the mechnical stress on your expensive iPhone/iPad.  Another difference from GJ2 is that our guitar input has much higher dynamic range – if you're into high gain models and tones, you'll definitely hear the difference.