There is no direct download link that works reliably in a browser for this OS. The only method is using the Mac App Store’s legacy mechanism.
: Temporarily disconnect from the internet and set your system date back to early 2012 via System Preferences > Date & Time . This makes the old installer's certificate valid again. Summary of Compatible Versions OS Version Typical Compatible GarageBand Version El Capitan (10.11.6) GarageBand 10.1.1 (or late 6.0.5 versions)