To download GarageBand macOS Catalina 10.15.7 for free, the most reliable method is to Mac App Store's "Purchased" history to fetch the last compatible version (typically version 10.3.5). How to Download the Compatible Version
If you have ever downloaded GarageBand with your Apple ID in the past, this is the easiest official way to get it. Apple Support Community Garageband For Catalina 10.15.7 -FREE- Download
If you are running macOS Catalina 10.15.7, you might have noticed that the current version of GarageBand To download GarageBand macOS Catalina 10