Help - out of gamut colours / borked palette?

[Ofnuts]

In the Lch model, for a given output gamut (sRGB here, I suppose) there are constraints between L,c,h. So what Gimp it telling you is that you can't just get a darker color (lower L) without altering c and/or h. If you change the dialog to HSV model (which is just a plain mapping of the RGB values), then you can change the V (which is close to the brightness, but not exactly the brightness) to darken the color, but of course you'll have a slight hue change in the Lch model.