I want to make a gradient with less unique colors, so I can do shading for messing about with pixel graphics, but as it is, GIMP's default settings create a very smooth gradient, which ironically is not useful in this case. I want to avoid lowering the color depth of the whole image if I can, however if that is the only way, that could work too. I mostly want a less smooth transition.
To do this you would define your own gradient with several sections, each section being a uniform color. However, if you want more than half a dozen segments this is going to be cumbersome to do wit the gradient editor. But since the gradient file is a just a plain text file, you can make one with a text editor and the help of your calculator or your favorite spreadsheet program (the GGR isn't very far from the CSV). The gradient file format is explained here.
One primitive way to do that is making the gradient on one transparent layer or new file, lowering the bit depth on this file and then copy it to your original file.