Createa new empty Dorico project. Apply the Silent Playback Template. Add an Alto Flute instrument to a Single Player.
In the VST and MIDI tab of Play mode, press the + button to add a VST plug-in.
Select Garritan > ARIA.
Click on the little ‘e’ button to open the VST window. Load the Alto Flute, and set up Reverb as you want.
In the Track Inspector, apply the ARIA player to your Player/instrument. You’ll also need to assign an Expression Map. Use either the GPO Default or GPO Woodwind map from here. (Even if you are just using the GIFF instruments.)
Now, back in the VST tab, next to the + button, there’s a little icon to Save an Endpoint. You can then save this as “Garritan Alto Flute”, and it will be available to add to your Playback Templates.