To get this achievement you must explore all of Los Santos and Blaine County. And it is worth 30G.

This achievement is obtained by "unlocking" all the map. You'll notice when you first start the game that most of the map is greyed out with "fog". You need to explore the whole map, to make it all visible. Unlike previous GTA games, the map is fully explorable from the start, just the pause menu map needs to be revealed. And when there is no fog left on the map the achievement will be unlocked, the easiest way to do this is on plane.

If you need help PM me.