Being a traditionalist sort of artist (I'm broke) I have had to make do with more modest means of getting a choir mixed into my arrangements. So for free (true, not a misprint) I located a couple decent choir soundfonts (sf2) and downloaded the Audacity audio program (it's a mixing console, not a host, with native versions for Win, Mac and Linux).
Then, I set up a folder, mute the vocals, and render out the Finale/GPO voices in aiff format using 'save special as...' If your arrangement is large, and you want to be fussy, save each part separately, or smaller groups of parts, as this will give you much better audio quality as a final product than the all-at-once GPO export. I use HP with ambience effects. Then I save the vocals as MIDI files (sometimes breaking up SATB if I want to edit in post) then convert them to aiff. I use QuickTime Pro and QMidi on a Mac to do this; there are Win equivalents. You need to change the preferences so these apps load your choir sf2 (not the default sofsynth), and the vocals in Finale have to be on channel 1 and set to Choir Aahs.
Post production is like a recording studio. Open Audacity (it's a bit slow, but at $0, it's $600 less than Bias Deck or any major host) and import your tracks. If your file names are long, you can rename the label on each track. You can sort your tracks after import as well. Your GPO sounds should all line up perfectly. Then I bring in the vocals...those will require a little tweaking to line them up. I leave the vocals dry (no reverb) as this results in clean alignment and minimal edits. You can add VST reverb to the overall result later if you want.
You can then add the full panoply of vst effects available as needed (be sure to download the VST manager when you download Audacity). Then export your final product as WAV, AIFF, MP3.