Gloomy means "dark and dreary." A cloudy day, a sad song about lost love, your downbeat mood after your team loses a big game — all of these can be called gloomy.
Have you ever been called a Gloomy Gus? If so, you must have been acting depressed or sulky. But you aren't the first to be called that — after all, Gloomy Gus was a comic book character who first appeared in 1904. By the 1940s, this nickname caught on, describing — and possibly adding to the misery — of those who are less happy-seeming than the people around them.