"Ghetto" is figurative in both your examples. I worry that your understanding is a little too neutral; the word suggests a low-class, close-knit community.
Honestly, I'm trying to find the "gay ghetto" quote in another place other than a dictionary site. Unless you can find the original quote, I'd leave that as an unanswered question.
For "in passing" vs "by the way", I don't agree that they are interchangeable. "By the way" is more like a comment in the sentence. "In passing" is part of the sentence.