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 -25 cents
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.