One thing to consider, when uninstalling GarageBand are the huge Instrument Library and and the Loop Library, also large Demo Song projects. On my Mac, where I am frequently using GarageBand, the Instrument Library contains 100GB of Sounds, and the Loop Library additional 50GB of Apple Loops. Uninstalling GarageBand will not remove these large Libraries, because they are shared with other music creation applications, like Logic Pro and GarageBand. You have to keep these large files, if you are using Logic or MainStage, otherwise you should remove them as well.
To remove MusicCreation files, the easiest way would be the Storage dialogue: See used and available storage space on your Mac – Apple Support (UK)
- Choose Apple menu > About This Mac.
- Click Storage, then click Manage.
Now look for the category "Music Creation" and check, how much content is been used by GarageBand, MainStage, and Logic. You can delete this Content right from this dialog.
But you should be aware, that you cannot necessarily get the content back, if you are still having loops and instruments from GarageBand '11 or GarageBand '09 or even older. The current version of GarageBand will only reinstall loops and sound from GarageBand 10; most of the beautiful virtual instrument loops and sounds from the older versions are missing. If you want to be able to reinstall the much more versatile loops and instruments from the earlier GarageBand versions, you have to make a copy somewhere to archive them, before you uninstall them.