The detail about the setting that best helps convey the mood of the excerpt is: he had failed in a more subtle ambition, and for the moment he was heartsore and disillusionized.
This detail encapsulates Gortsby's feelings of defeat and disillusionment, which are central to the mood of the passage. The contrast between his potential to join the vibrancy of the "thoroughfares of light and noise" and his current state of mind emphasizes his melancholy and introspection, contributing significantly to the overall atmosphere of the excerpt.