The most common and most likely answer to this crossword clue is the 10 letter word DEPRESSING
I've seen this crossword clue in The Guardian
D
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Buddy explains!

The straight part
We know that "Gloomy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gloomy = depressing.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DEPRESSING mean?
DEPRESSING verb- To press down.
- To make depressed, sad or bored.
- To cause a depression or a decrease in parts of the economy.
- To bring down or humble; to abase (pride, etc.).
- To reduce (an equation) in a lower degree.
- Causing depression or sadness.
- Causing a reduction in economic activity.