As you've discovered, GB can be a very "bloated" application.
If you want to make music, there are other, better apps available.
As for getting rid of all the unwanted files...
I'm thinking you're going to have to do "a search" to locate them, and then delete what you think you can get away with deleting.
The "search tool" that I recommend you use is
a small free app named "EasyFind".
You can get it here:
A little collection of free utilities for the Mac to make your life easier.
www.devontechnologies.com
Set up EasyFind like this:
Search for - files and folders
Operator - all word
Comparison - ignore case
Include - invisible files and folders
Then, enter "garageband" into the search bar and hit the "magnifying glass" icon on the right.
Give the search some time to complete. BE PATIENT.
The "hitlist" will display files and folders.
It will be UP TO YOU to decide what you want to try to get rid of.
What you can do is... click on an individual hit in the list, then RIGHT CLICK on it and choose "reveal in finder".
If it's a folder, you can then do a "get info" on the folder to see how much space it occupies.
If it's full of stuff, again, it's UP TO YOU to decide whether to delete it or not.
I would go into the folder, and explore first.
Good luck.