From The Art and Popular Culture Encyclopedia
Group rape (also known as "grape", "gang" or "pack" rape) occurs when a group of people participate in the rape of a single victim. 10% to 20% involve more than one attacker. It is far more damaging to the victim, and in some jurisdictions, is punished more severely than rape by a single person. The term "gang bang" was a synonym for gang rape when public discussion of sexual activity in general was taboo; in the advent of the pornography industry and relaxed sexual tensions, that term is now often used as a slang term for consensual group sex.
This is also related to rape as means of warfare, where the pack mentality is highly predominant.
According to sexual crime profiler Roy Hazelwood, gang rape "involves three or more offenders and you always have a leader and a reluctant participant. Those are extremely violent, and what you find is that they're playing for each other's approval. It gets into a pack mentality and can be horrendous."