Gif save a limited number of colors, and the original image already have some problem due to the JPG compression: reducing colors let it more evident. 1) Select the sky area with the problem before exporting in GIF 2) Blur the area to contrast the JPG artifact, p.e. you can use Reduce Noise or Surface Blur 3) When export in GIF, use Export/Save for web, and add Dithering , this will help to have a kind of gradient even if not all the colors are there. PS you cannot have a good quality with gif, it can store at least 256 colors, not more!
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