Just wondering if I'm missing something - but I thought that F12 was a "GOG Galaxy Screenshot" function - one that would save a Shareable Screenshot within GOGg no matter what game you were playing in it...

Did something change?
I looked in the Settings and there is no Shortcut Key configuration for it.
I looked in the forum but noone seems to be saying anything about this specific query (F12, or F12 not working anymore, etc)

Thanks for any input!

It does not work in all games; overlay support needs to be present for the screenshot function to work. The older the game, the less likely it'll be able to offer that option.

Ah ok, I wasn't seeing any notification or anything so that must be it - thanks for the info!

Also, note that RivaTuner doesn't really play nice with the Galaxy Overlay. Running RT at the same time as a game with Overlay may result in the Overlay functionality being turned off.

The game cards list things like overlay support. Of course, in order to read the card, it must still exist as GOG removes the cards for removed games.