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The gangs aren't always around...sometimes only at certian time of day. A couple near water's edge on the Eastern islands can be a real pain to wait for. Just keep visiting likely places until you get one to show up.
Aside from the map, which is great for finding their turfs, a trick that I learned was that since some of the gangs are only active at certain times of the day (such as only at night), switching between Tony and your Enforcer comes in handy from time to time as that will advance the clock by six hours every time you switch. It helps save time waiting around for them to show up, and both Tony and the Enforcer are great for laying the smack down on the gangs.