Start a default new project. You'll get a Piano track. Get the track info by double clicking on the track (piano) icon or pressing the ? + i keys.
On the bottom right you'll have access to the instrument's Details
From that section you can change the instrument generator to a third party plug-in such as Garritan ARIA Player.
Once the Plug-in is chosen, click on the edit button to show its interface and load an instrument you want to play
Alternate method for GPO4 users
Fellow user Rick Schneblin prepared CST files (Software Instruments Presets) which allows GarageBand users lo load GPO4 instruments as software instruments presets.
Installation instructions are enclosed in the zip file.
Tags: 04, 05, 06, 08, Apple, band, Garage, garageband, osx