I'd be very surprised if this were the case: the bundled GIFF instruments were a subset of the other Garritan libraries; and none of the box sets contains a Theremin, from what I can see. I can't see a Theremin in my copy of the F2012 installer.

I seem to remember instruments being added at some point, but not being taken away.

If I add a Theremin in the Setup Wizard on F27, I get a drawbar organ as the sample. Perhaps you were using something a bit more Theremin-y, like the Children's Choir?

If you want to make absolutely sure, you could install 2012, and then reinstall the latest ARIA Player. That should keep all the installed instruments, but with the up-to-date Player software.

Already installed.  Theremin works great, esp with Smart Shape gliss.  (Make sure using VST, or you get the GM sound.)

Can you tell me where the Theremin is in the GIFF instruments?

Here's the Score Manager (note that the hieroglyphs are 2012... sorry; inside joke).

In "Add Instrument" it was Electric/Keyboards/Theremin.

I just want to find out how to make the 2012 GIFF instruments a VST that I can use in v26.

I'm assuming that I have to treat it like a 3rd party plug-in.  I was just going to copy the filepath from 2012 for the instruments, but the "Manage VST Plug-ins" doesn't allow copy/paste.  What do you do?  Copy it by hand?

By the way, I haven't been able to get the Theremin to work in my 26 version.  It says it's a "Lead 6 (voice)", and on the ARIA Player it says this:

The first picture shows Finale's Instrument List, which is just "names for staves". It doesn't mean there is necessarily a Theremin instrument in GIFF. 

The Garritan Instruments for Finale are sample instruments controlled by the ARIA Player. Finale send MIDI data to ARIA, which plays the samples. 

Finale 2012 uses the ARIA Player as a VST, same as Finale 27. You should be able to access the ARIA Player in 2012 in the same way as in F27. 

If ARIA can't find the Lead 6 Voice (which I presume is the actual sample used when Finale creates a staff named "Theremin") -- then you need to reinstall the GIFF samples for Finale 27. 

The only place I find the Theramin (in Finale 27) is in the SoftSynth voices, not in the VST resources.