The Gang Operations are represented by an icon of a fist. After completing a Gang Operation, you can see that portions of your map have turned from grey to purple.
You can tell where Gang Operations are located by bringing up your map and heading to areas that are still seen as grey. However, the icons for some Gang Operation missions will not be see on your map so you have to fly or drive around the grey areas of your map until you see a large group of gang members in one area. Kill them all and it will count as completing an Operation.
Note: The Gang Operations can be completed easily after obtaining aerial vehicles with mounted weapons.