Does this file exist:

D:/Garritan/Instruments for Finale/Programs/Include/gpo_curves.sfz

(note that ../ means go back one level.)

Also, that is a file path for the instruments that come bundled with Finale, not for GPO5. 

No, it doesn't. I don't even have the Include folder.

If you don't have the Include folder, then you should be getting error messages for lots of instruments, because they all rely on it. So it's weird if that's not happening.

Also, are you sure you're selecting the GPO5 Timps, not the GIFF timps? 

Either way, I'd suggest re-installing GPO5 and the Finale GIFF instruments. 

There's only one option for Timpani in the Aria Player when I use it under "Garritan" in the score manager: "Timpani KS". I don't know which one that is. Where can I re-install the stuff? EDIT: wait, found the Include folder. But the file in question isn't there. I know I didn't delete it.

And what's the path of the Include folder?

It sounds like you only have the Garritan instruments that are bundled with Finale, not the separate GPO5 purchase. 

I'd reinstall the instruments from your Finale installer. If you only have an old version of Finale, you may want to update the ARIA player afterwards.

The path is D:/Garritan/Instruments for Finale/Programs/15. Instant Orchestra/Include

Yeah, I just have the latest version of Finale. I also have the latest version of ARIA.

That's a separate, different 'include' folder for Instant Orchestra, not for Garritan Instruments for Finale.

Well, that's the only Include folder I have.

Then you need to reinstall the Garritan Instruments for Finale, using the Finale installer. 

Used the upgrade from 26 to 27 setup (already had 27, but didn't have the regular install file) to re-install everything but now I can't open Finale. Apparently it deleted the .exe

If the installer for Finale left you with no actual copy of Finale, then I suggest you raise a support request directly with MakeMusic, for them to figure out what's going on on your system.