Get in a Rustler from the abandoned airport and fly southeast. Once you pass Los Santos International Airport, fly to the top of the second altitude bar and release A to allow the plane fly in autopilot. About every three minutes, look at your stats and examine the Gang section to find that the Vagos or the Ballas have gained a territory. Sometimes the Grove Street also takes more territories. To get the entire map filled with gang territories, let the plane autopilot for over two hours. Check you plane periodically so that it does not crash in the ocean. Once you die, the territories will be visible on the map.