Once you've added a new track to your project, you can edit the track input to make sure GarageBand records the correct device. The track input determines where the specific track's instrument input is within GarageBand. This is different from configuring the hardware and software input.
Select Track
After you have a new track in your project, double click on it to open the right sidebar.
Edit Track InformationOpen the track information by clicking on the "i" button in the bottom right corner.
Select the Input Source that your instrument is plugged into. GarageBand will give you a list of all the applicable sources on the computer.
If you want to hear playback of the instrument, turn the Monitor to the On position. You can turn it Off if you don't want to hear it.
Once the correct input is selected, you will see the instrument's signal in the track when you begin playing.