Hi Fabrizio,

I am curious what version of GPO are you wondering about? GPO 5? GPO 4? Or earlier? Garritan Personal Orchestra 3 and were managed and supported by Native Instruments. You can find their website here. 

As far as Garritan Personal Orchestra 4 & 5 - I expect both products to work on macOS 10.12 Sierra with the latest version of the ARIA Player. You can find the latest version of the ARIA Player here.

Hope this helps!

GPO 5, latest update, bought yesterday. The standalone player works, but the VST is not recognized/loaded by any DAW (Live, Logic). 

Apple made some changes to the security system, so if the developers have not addressed explicitly this issue the older version of VST will not work. Other VST (like Sylenth) have gone through this same error. Many vendors have released Sierra explicitly compatibility announcements (https://help.ableton.com/hc/en-us/articles/211499129-macOS-Sierra-compatibility).

I downloaded that player and the AU plugin is recognized and works. But not the VST version.