Did you scale it down after the color/levels processing? Due to the limited it-depth of Gimp color channels, when you do "heavy" color processing in Gimp, you lose colors or, more accurately, you lose levels in the color channels(*) sometimes as much as 10-15% of these. The result is a light quantization of the image, very often showing as '"banding". The corresponding histogram shows regular gaps at one end and spikes in the other ("haircomb" histogram). Scaling down the image afterwards makes that a bit less noticeable because the pixel interpolation smooths everything.
The cure is to use a high-bit-depth-capable application (plenty discussed on pixls.us) or the next version Gimp (currently experimental 2.9.x), that supports 8/16/32-bit and floating-point channels.
(*) All color processing is mapping the 256 existing values into other values. If you add the constraint that the output should keep the values in the same order, the only way to keep 256 output values is to do nothing. Hence all other processing produces less output values.