MakeMusic are still selling Garritan products, and the Finale announcement made no mention of them; so you should still be able to get support directly from MM. (Use the "Contact Us" link at the to...
Seriously, it's 22 year-old software. Even if you could register it, you'll be lucky if it runs on W11. Even the latest version, GPO5 is nearly 10 years old.
Just download the GPO5 installer from your account. GPO5 is just GPO4 plus additional instruments. If you're on a Mac, you may need to download the latest ARIA Player from this Help Center.
You may still get support for issues concerning registration and authorization, so it's worth submitting a support request to MakeMusic.
As my doctor says: What seems to be the problem?
If you actually want help with Dorico, then I'd strongly recommend using the Steinberg Dorico forum. You'll find many former Finale users there, who may be familiar with the problems you're having....
What happens if you load a CFX instrument into a slot in the ARIA Player? (E.g. clicking on something in the left-hand panel, under the CFX header.) What version of ARIA are you using ? (Shown in t...
I would contact MakeMusic. They do still offer support for this.
Win: https://store.makemusic.com/Downloads/Default.aspx?id=648 Mac: https://store.makemusic.com/Downloads/Default.aspx?id=649
You should find the installer in your MM account.