The Desaturate submenu contains operations that turn an image into a Grayscale representation, using different methods.
Kohta 8.29, ”Color to Gray”
Kohta 8.30, ”Desaturate”
Kohta 8.24, ”Mono Mixer”
Kohta 8.32, ”Sepia”
You can access this command from the main menu through Colors → Desaturate.
