smooth out colours

[Ofnuts]

I don't see anything abnormal here; This is 8-bit per channel imagery. There are round-of errors and limitations. So yes, there is is banding in your cloud/sky picture but you really have to put your nose on it to see it. You can mitigate it using Filter>Noise>Spread.You can also use Gimp 2.9.x and do the image in 16 or 32-bit mode (but look out for surprises when you export the final result, because that will be 8-bit...).