Sorry about the bug with FFT. It should definitely allow you to pass a list to it!
As you noticed, the Macintosh GP app has the "lib" folder embedded in it. This makes it easier for Mac users to move, copy, or delete the GP app; the app contains everything needed to run GP (although it doesn't include optional parts such as the instrument library and the sample projects).
However, you can still explore and modify the .gp files in the lib folder. Just control-click on GP.app and select "show package contents". This will open the .app file (which is actually a folder). If you open a few levels of nested folders (Contents/MacOS/runtime), you'll find the lib/ folder with all the .gp files inside it. You can edit those and restart GP to see your changes.
Obviously syntax or logic errors can break GP, and it's a bit harder to debug such problems because the errors go into a log file, not into a terminal window (since there is no terminal window). If you plan to do a lot of hacking of the internal GP code, you could copy the entire runtime folder to your Desktop and rename it something like "Experimental GP". You could then open a Terminal, 'cd' to that folder, and run GP from the terminal like this:
You'll get the command prompt:
gp>
You can type "o" to the prompt to start the GP project editor. If you encounter errors, you may see some useful information in the terminal.
Note: This is the process we've used to bootstrap GP. It won't be documented or supported and eventually it will be replaced by a debugger, browser, and other mechanisms that let you modify GP from within itself (as you can do in Squeak). GP just hasn't become entirely self-supporting yet. That's one of our primary goals for this next year.