How many guys on one person qualifies as a gangbang? This is an important scientific question. Obviously two just makes it a threesome, but where do we stop calling it a "some"?
I think the major difference is more in consent of the bangee(s) then number of bangers. Though, I think a gangbang probably has to have at least 3+ people for them to count as a “gang” of people non-consensually banging someone.
Though to answer your last question I think 5+ people just counts as a(n small) orgy.