I think Jackal has a valid point, Dago. A gang could be thought of by some as a group that gets together to solve crime (Scooby Doo) or it could be a group of people with common interests that socialize (motorcycle gang) OR it could be a group of people who promote violence to protect themselves (crips, bloods, etc). It really just depends on your personal definition, not what a book says.
From Websters
gang1 /gæŋ/ Pronunciation Key
–noun
1. a group or band: A gang of boys gathered around the winning pitcher.
2. a group of youngsters or adolescents who associate closely, often exclusively, for social reasons, esp. such a group engaging in delinquent behavior.
3. a group of people with compatible tastes or mutual interests who gather together for social reasons: I'm throwing a party for the gang I bowl with.
4. a group of persons working together; squad; shift: a gang of laborers.
5. a group of persons associated for some criminal or other antisocial purpose: a gang of thieves.
6. a set of tools, electronic components or circuits, oars, etc., arranged to work together or simultaneously.
7. a group of identical or related items.