hmm wait, i think i get it. you have a gradient from one color to the other, say dark blue to light green, and let's say you have a greyscale drawing made with 10 different shades of black/white, from pure black to pure white. the gradient map assigns the dark blue to the black pixels, and so on, until it assigns the light green to the white pixels. so it turns the greyscale into colored using the brightness values of the pixels to correspond to colors on a gradient