nonamethanks said:
What's the difference between gangbang and threesome?
The key difference, as I understand it, is that in a gangbang situation, one person has sex with multiple other people who do not sexually interact with each other. If any two individuals apart from the one who is the central focus are having sex, it's not a gangbang. See the Wikipedia article on the subject. As an example of this distinction, post #2903772 would not count as a gangbang, but post #2160946 would.
The rule against using the gangbang and threesome tags together dates from a time when the tags' usage was based on the number of males and females involved. Since the creation of the *_threesome tags, the gender makeup of gangbangs is no longer relevant, so I would consider this rule obsolete.