Hallo dOrr, welcome to the forum. You are presenting us some really nice photos here, and like everyone else, I also like the first best of the series. That little moon is just such a nice little extra!
Sorry, I don't know the answer to your question about the gradient you are referring to, though I do see what you mean in each of your photos (save the second). I would not even know what could cause it, let alone how to solve it. Sorry.