Gradient Map from the Image menu re-paints the selected area of an image in the chosen gradient pattern by matching contrast to the pattern. It’s a bit like converting an image to shades of grey, assigning a value to each pixel between 0% and 100% according to its shade and then taking the color from the gradient bar that is at that percentage along the bar from left-to-right.
A simple use of Gradient Map is to wash an image in a foreground color. For example, set the Foreground color to red and Background color to white (using the color selection buttons), set the gradient type to "FG to BG RGB" (foreground to background red/green/blue) and then click OK.
An inverse effect is created by selecting the Reverse checkbox (the gradient bar is used right-to-left instead).