Umm - i'm using GPO4 with ARIA and Finale and it sounds fine - now, if i want to detail things in cubast studio5, - well, how do i get the music and the sounds into cubase?
I don't use either, but the general approach to this kind of question is: what language (file format) do both programs understand?
Option 1. You can export a multi-track midi file from Finale and import it in Cubase, and then set up the instruments on the channels in Cubase like you did in Finale. You might lose information though, but you'll be able to tweak the notes.
Option 1a. If MIDI export doesn't work, you could try to have Finale send the MIDI data to Cubase (in real-time). Make Finale the master clock, set Cubase to record and hit play in Finale. Note that you can probably only export 16 tracks at a time. Once it has been copied, you set up the instruments in Cubase and ready you are.
Option 2. Search for another format they have in common, e.g. MusicXML (see this thread).
Option 3. You export audio from Finale, and import it in Cubase. You can do that track by track, or the stereo mix, etc.
Perhaps there are people with more knowledge of both packages, and they can supply more options, but this is it, I think.