Gang Attacks will only unlock once you have reached Rank 18. After hitting this Rank, you will receive a phone call from Martin informing you of local gangs hanging around various different neighborhoods in San Andreas. Your goal is to terminate these gangs from their hideouts.
Gang Attacks are indicated by large, yet faint, red circles on your mini-map. Take note that these red circles DO NOT appear on the world map, so keep your eyes peeled on the mini-map when cruising around Los Santos and Blaine County. Once you enter the circle, the Gang Attack will immediately begin as AI gang members start attacking/ambushing your position. You will need to eliminate the entire group of gang members - ranging anywhere from 15 to 30 enemies - until the area is clear. Simply survive the entire assault without dying and get at least ten kills in order to unlock this achievement.
While this can be done on your own, it would be beneficial to work on this with at least one other player. This is helpful because your partner can take all the damage and clear out the rest of the gang hideout once you have obtained your ten kills. Remember, the achievement will only unlock once the Gang Attack is complete, not when you get your tenth kill.