As a simple experiment, I took the instructions in the tutorial on the GimpTalk forums and created a script-fu for making PS-style gradient maps. The steps to do it are pretty simple, but I added a few new options to it, such as flattening image and layer opacity.
Works quick and should also be simple enough to work in either 2.2 or 2.4 without any problems. I have built and tested this in 2.4.5. If it doesn't work in the older versions, let me know and I'll try to work out whatever bugs come up.
Oh, and the original tutorial this is based on can be found here: