There are different types of Neutral Gangs on the Map.  Each of the Neutral Gangs has a certain strength level, and they also hold specific types of Rewards.  If you find a gang that you think you can defeat, just click on the gang and send your Troops to attack.  If you win the Battle, you will eliminate the gang from the Map and collect any Resources or Items they were holding.  You must start with defeating Level 1 Gangs of a specific type, before you can attempt to defeat the next level of the same type, and so on.