If you installed loops - you'll have to clean them out: https://support.apple.com/en-us/HT201503
If you made projects, they could be stored anywhere on the disk - probably in a /Users folder.
I had the same problem if the people above did not help, take these steps:
Download OmniDiskSweeper (a comprehensive file removal app) from the App Store.
Open the app and click your main drive.
Give the application about 5 minutes to size your driver
GarageBand and the loops should be under the same folder I'm pretty sure its either "Users" or "Applications", click the item you would like to delete and press the "delete" button on the bottom right of the application.
You can also use this app to delete unnecessary files or garbage files. Please try not to delete any files that seem unfamiliar to you because you could harm your computer integrity! I do not claim any liability or responsibility for any mistakes or files you may accidentally delete!
Try removing (or moving) files from the following locations:
~/Library/Application Support/GarageBand(user's files)
~/Library/Audio/Apple Loops(user's files)
You can also try locating the files by
locate command, e.g.:
Note: To rebuild db for the first time, run: