Gangs sometimes become dangers to their communities because they can have large and strong memberships and are the most organized entities in those communities.
Particularly in impoverished areas, the gangs tend to have more resources than any other organization. They have weapons and the willingness to commit acts of violence. Because of this, community organizations, if any, are more or less powerless to control the gangs. The gangs can intimidate anyone in the community. In addition, their violence can be dangerous to those in the community who either A) try to resist them or B) are simply unlucky enough to be caught in the crossfire.
Gangs can continue to be dangerous because police lack the resources to prevent them from flourishing. It is very difficult to suppress gang activity by police work.
Thus, gangs are more powerful and more violent than any other group in many communities. Therefore, they can be a grave danger to those communities.