What exactly is the problem? Are the VSTs not appearing in Cubase and Dorico? Do you have the latest version of ARIA Player and Engine? This includes a 'VST3' version, as well as the 'VST2' plug-ins. The VST2 plug-ins must be allowed explicitly in the preferences.

Once you've got the VSTs available in the apps, then you need Expression Maps for Dorico to translate the notation into the MIDI signals that Garritan wants.

I've written some instructions and provided Expression Maps for GPO5 here:

https://www.steinberg.net/forums/viewtopic.php?f=246&t=201477

There are some old ones for Cubase here:

https://www.steinberg.net/en/support/content_and_accessories/expression_maps_for_vst_expression.html

There is new documentation for Dorico - en Français! 
https://steinberg.help/dorico-manuals/dorico-pro/dorico-pro-3/