What a great and sophisticated question! The best answer that I know of comes from Mary Douglas, the great cultural anthropologist. Her basic point is that dusk is between day and night. It is liminal. In other words, things that are liminal defy classifications. For this reason, often the human mind or people in general fear these liminal areas. So, if a movie or novel sends the defeated into the dusk, they are being sent into the unknown - outside of our classifications.
There is another side. The liminal areas can also be the areas of people who are extraordinary, such as heros. I highly suggest that you read some of Mary Douglas. She is wonderful. I will link a book review that I wrote of her, Natural Symbols.
The significance of dusk is entirely created by Gortsby, who is naturally drawn to such a time of day, which he associates with being the "hour of the defeated." He looks at this time through his own eyes and out of his own situation, which, as the story tells us, finds an echo with his own failure as Gortsby as well counts himself among the "defeated." Dusk then, to Gortsby's mind, is a time when those who have failed in life in whatever way can be free to come out without being noticed and talked about by others. Dusk provides such people with an anonymity that it is suggested they crave.