I want to fill a rectangle with a continuum of colors. I want the color on the left to be black and the color on the right to be white. In between, there are different shades of gray.
This part is similar to the already answered question Color gradient in tikzpicture from top to bottom corner.
I also want there to be several vertical arrows with small labels like "A", "B", "C", and so on. These arrows are below and above the rectangle and do not extend to the borderlines of the rectangle. In the following figure, I have created an example of how I want the rectangle to look.
The rectangle should be an independent object. It should not have an axis.
What could a minimal worked example look like that uses the packages tikz/pgf?