Need Help & Advise

[bornfree]

using RAW file format or jpeg?

is it due to stray lights from nearby street lamps? maybe use a hood?

[Sivakis]

I had this happen to me before also. Although I cannot confirm, it was either because of stray clouds in some of the pics or from street lights. Near-horizon shots also tended to give me that problem.

[ivan]

This is called posterisation, which happens when you are not processing in 16 bit and above. GIMP only handles 8bit, which truncates the original 32 bit. By reducing the bit depth by such a level, you will get this upon stretching. You'll need to use another software that can handle 16 bit, such as lightroom or photoshop.