What letter do you change in the word detection to make a with definition of gloominess?

You change the "d" to a "s" to make the word "section" which means a part or division of something.

While "section" doesn't directly mean "gloominess", it's the closest match to the original word.

There isn't a single-word answer that perfectly fits the puzzle, but perhaps you were looking for a more figurative answer? For example, you could say "dejection" which means a state of sadness or depression, and could be considered a form of gloominess.