Photoshop :: How To Gradient Remap?

I have an laser scan which is of regional elevation. The information from the scan is essentially displayed as a custom gradient, that I have no control over. I would like to change this gradient to a much more logical black is low and white is high. The problem is that the original gradient has every color with variations of lightness so using something as simple as gradient map can not work. What I would essentially need to do is define an input gradient, and define an output gradient, and then let photoshop calculate it. otherwise, I will have to go through and do custom selections for every color and then manually remap the color. here is a good example of the data.

weather(dot)unisys(dot)com/usgs/legend(dot)gif

The gradient needs to be converted black on the left and white on the right with all the range in the middle corresponding the appropriate grays.