Gang attacks can be found around the whole of the map. They are identified by a red circle on the map, and are only visible when you are close to them. You will likely come across the one that spawns at Trevor's airbase the most. There are plenty to find, and they will being to appear after you reach level 18.
Only a limited number of gang attacks will be available at any one time. Each group of locations will only be active for eight hours of the day (4am-12pm, 12pm-8pm & 8pm-4am). The one at the airfield can be found between 12pm and 8pm, but there are plenty of others to choose from. The map below shows several of them, and the times at which they can be found.
Upon entering the red circle, you will be attacked by gang members. There will be between 10 & 30 to kill. A counter will appear in the lower right of the screen. To get this achievement, you must kill 10, so it will be easier to do solo. Attempt to find cover, or invest in body armour if you are struggling. Aim for the head, and kill them as quickly as possible. Keep checking your mini-map to be aware of any coming up behind you.
If you are killed, then you will not be able to unlock the achievement, and you will need to wait for it to respawn, or find another one.
Successfully completing it will award you with 500RP (more if you are playing with more players), and the achievement, assuming you didn't die and killed 10 or more enemies.
I have included a video guide for anyone who prefers. It shows the tactics used to complete the one at Trevor's airfield. The achievement was unlocked on the Xbox 360, but the information is the same across both platforms.