To import an iTunes Match song, you must first download it to your device:
- Open the Music app.
- Tap My Music.
- Tap Artist, and then select Songs to change to songs view.
- Find the song that you want to download. Click the More Options icon , and then select Make Available Offline to begin the download. The song downloads to your device.
After the song downloads, you can import it into GarageBand.
Apple Music tracks can't be used in GarageBand.