Hi - Are you talking about a real guitar with its audio output plugged into the interface? If that's the case then the answer is no, you can't use that to play GarageBand's internal Software Instruments—you'd need a MIDI controller [like a MIDI keyboard connected via USB] to play internal GarageBand software instruments [piano, guitar, drums, bass, etc] on Software Instrument tracks. Once you've gotten a clearer understanding of the difference between MIDI/Software Instrument tracks and Audio tracks then there's a longer answer but it would involve more advanced equipment and applications, and I'm not sure if that's really what you're asking..