This is an example of a simile, a comparison between two things using the words "like" or "as." "Gloomy as the night" is the simile here, since his sadness is being compared to nighttime.
It means to give credit to the author and source. It helps in an argument because when you're trying to prove a point, you have to have factual evidence that proves you're right.
What? Do you have a question?
