General Antonyms:
* Joy
* Happiness
* Cheerfulness
* Brightness
* Light
* Hope
* Optimism
* Cheer
* Exuberance
* Vivacity
* Enthusiasm
More specific antonyms, depending on context:
* Sunlight (if referring to darkness)
* Prosperity (if referring to economic hardship)
* Success (if referring to failure)
* Peace (if referring to unrest)
The best antonym to use will depend on the specific context in which you are using the word "gloom."