Both Uses

prosperous
in
The Great Gatsby
(Edited)

The interior was unprosperous and bare; the only car visible was the dust-covered wreck of a Ford which crouched in a dim corner.
p. 25.1

unprosperous = not looking successful or good

standard prefix: The prefix "un-" in unprosperous means not and reverses the meaning of prosperous. This is the same pattern you see in words like unhappy, unknown, and unlucky.
I was immediately struck by the number of young Englishmen dotted about; all well dressed, all looking a little hungry, and all talking in low, earnest voices to solid and prosperous Americans.
p. 42.2 *

prosperous = financially successful

Definitions:

(1) (prosperous)
successful or good -- especially with regard to finances or wealth
(2) (meaning too rare to warrant focus)