The Garritan installers contain executables, notably to decode/decompress the .data files into .audio files. These executables are likely 32-bit and no longer supported on newer Mac OSes. GPO4 was released in 2009. 

If you're planning to keep using Garritan, then I'd suggest buying GPO5, which should still run on Tahoe. GPO5 is GPO4 plus some extras.

If you have GPO4 installed on another Mac, you could copy over all the relevant parts: the Bank files, instruments, and GUI files; the samples; and also the plist files in /Library/Preferences for GPO4 and plogue.aria. 

Of course, there are many better (and better supported) libraries these days, such as Noteperformer, OT's Free Berlin orchestra; Iconica Sketch, the No Budget Orchestra, etc, etc...