6.0.5 is a 32-bit application. While it runs beautifully on macOS Mojave and earlier, it will not run on macOS Catalina (10.15) or newer, as those operating systems dropped support for 32-bit apps.
Apple’s official stance is that you can use their loops and sounds for your own commercial music royalty-free, though you cannot sell the loops themselves as standalone samples. How to AVOID Copyright Claims (when using Apple Loops) garageband 605 download top
Despite being "entry-level" software, GarageBand has a "top" reputation in the professional world: How to AVOID Copyright Claims (when using Apple
Because Apple no longer hosts the 6.0.5 installer on their official servers for new downloads, users resort to third-party archives. This is risky. : If you have a Time Machine backup
GarageBand 6.0.5 was the final release before the software underwent a major architecture shift.
: If you have a Time Machine backup from an older Mac, you can manually restore the application from the /Applications folder, along with its support files in /Library/Application Support/GarageBand /Library/Audio/Apple Loops Third-Party Archives : Community-maintained sites like Internet Archive