GarageBand is a bit limited compared to Logic Pro X, but there's a hack that you can use. It's sort of cumbersome though, but it should work. Essentially, you're going to create 2 software instruments and add the AUPitch plugin to one of them detuning it by a quarter tone (up or down, doesn't matter).
- Create 2 tracks (of the same instrument)
- Move to one of the two instruments and click on the
smart controls button
- Click on the
i to show the inspector
- Scroll down on the inspector until you see
Audio FX and click on a slot to add an effect
- Go to
Audio Units -> Apple -> AuPitch
- On the
Pitch control, double-click where it says
0 cents and change it to either
25 cents or
The "cumbersome" part of this method is that you'll have to record notes separately on each track and then play them together to get the desired effect.