Sorry for all the replies, but I wanted to pop a fix here in case anyone else runs into a similar problem.
(only a Windows fix unfortunately) once the game is running, hit SHIFT+G and there should be options for RENDERERS. Mine was by default set to "Automatically Choose". I have now set it to one of the options and its fixed.
Yikes! That is.......so bizarre!
But thank you for posting the solution. "Automatically choose" is the default selection for Ren'py and it causes issues for me as well - though nothing like what you saw. @_@ I just get weird flickers and flashes of dark areas.
This happens in every Ren'py game for me - not just Gilded Shadows. I always have to set the renderer to a specific one when I download a new Ren'py game.
O_O
