make a photo of sheet music black and white, remove grey

[rich2005]

(11-19-2021, 02:27 PM)carljong Wrote: Tried it.  Didn't do anything.

Well that is no help as a response.

Unlikely you will get g'mic plugin working with macOS 

If you can see this animation, it goes through the usual  https://i.imgur.com/5wNKaBW.mp4
auto - which does not work for you
Colors -> Levels and pick the white point
Use the Colors -> Curves

How are you taking the photos ? Mobile phone ? or camera ? Make a clip of part of the image and post it.