Apple's free music creation software lets you host Virtual Guitarist, Virtual Drummer, and Beatmaker as an AU plugin, this is a bit tricky because you have to go some extra steps.
1. Choose GarageBand > Preferences…, then Audio/MIDI > Effects and ensure Audio units is checked.
2. Select a Software Instrument track or create a new one. (Track > New Track… > Software Instrument and click Create.)
3. Select the track and click the Smart Controls button on the toolbar (the one that looks like a knob).
4. Click the ‘i’ icon in the Smart Controls panel. (The “Inspector”)
5. Look for the Audio Units section – you may need to scroll or resize if it isn’t visible.
6. On the right, you’ll see areas labeled AU Instrument. Choose UJAM and in that menu pick the plug-in you want to use.
When you authorized your plug-in with another DAW, it will come up in Trial Mode when you open it in Garageband.
This is a known Garageband issue, but you can easily resolve it for yourself: Just re-enter your credentials (re-authorize) while in Garageband, and all will be fine.