As you note, the verb "bear" is sometimes used to describe the act of giving birth. It is most commonly used in the past tense and in rather formal situations. If we recognize that we are discussing a work of fiction, set in the South, about that society's upper class, I feel this is just use of "artistic or poetic license" by Mr. Fitzgerald. In revisiting your source, you can see that it matches the surrounding paragraphs (omitted in your post...) by his slight alteration of the conventions of grammar or language. Even the quoted passage, while grammatical correct, is a tortured read (at least to me as a native speaker!). As you had suggested, the word "in" could be left out, or, an "a" could be added, or, it could simply be, "borne new life," but that was not what was written by the author. In short, as an ESL student, you should not be overly concerned about correcting the grammar in works of fiction or poetry. I hope this helps you! J.

to bear can mean to hold and carry, so "borne in" can mean "carried in". It is saying that, as darkness came creeping in upon the city, it "carried in" new life with it.