The detail about the setting that best helps convey the mood of the excerpt is:
"he could have strolled into the thoroughfares of light and noise."
This detail highlights the contrast between Gortsby’s current state of gloom and the potential vibrancy of the surroundings, emphasizing his feelings of defeat and disillusionment. The mention of “light and noise” suggests a lively and prosperous world that he is currently distancing himself from, enhancing the somber mood of the excerpt.