If you installed in the Default location, there should be a GPO5_Samples folder located in (drive)/Users/Shared. If you installed it anywhere else, you should be able to find it. If not, go ahead, re-download and reinstall. You probably won't have to re-do the license key.
I'm running GPO5 on my Intel Mac over Monterey 12.2.1 and 27 just found it. I did have some issues with 27 not putting a couple of files in the correct directory, however, and this affected NotePerformer but not GPO5. Using the Submit a request link at the top of the page, MM Support got me straightened out quickly.
The Finale installer is not designed to look for GPO5 or other Garritan libraries during installation but rather the Garrtian installers look for Finale. Since GPO5 was installed before v27, GPO5 does not know that v27 is there.
In this case, there are two options to quickly get things setup. One options is to run the GPO5 installer over your current install and this will place the necessary files where they need to be so that v27 sees and connects with the library.
The other option is to follow the steps outlined in the knowledge base article here. This article describes how to manually copy these files where where Finale needs them to be. Either option will get you the same results.