The detail about the setting that best helps convey the mood of the excerpt is: "on the bench by his side sat an elderly gentleman with a drooping air of defiance that was probably the remaining vestige of self-respect in an individual who had ceased to defy successfully anybody or anything."
This detail reflects the somber and defeated mood of both Gortsby and the elderly gentleman, highlighting their shared experience of disillusionment and the passage of time where self-respect has waned, contributing to the overall atmosphere of melancholy and resignation in the scene.