Start a default new piano project (it's an example as it's the easiest way to start). You'll get a Piano track. the software instrument info should be visible in the pane on the right. If not, by double click on the track (piano) icon or press the ? + i keys.

Click on the edit tab located on the right.

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 Sound Generator 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.