First things first: people are arguing over whether this is actually a map editor, but that's what the creator—Guadmaz—has called it. From what I can tell, this mod doesn't allow you to change the layout of the world in GTA V but does let you spawn objects like ramps and vehicles into that world.
The creator compares it to the Multi Theft Auto: San Andreas map editor, so if you've used that you should have some idea of how this will work. Here's the description:
You have a freecam to navigate around, and you can spawn objects (with preview!), move them around with your mouse or keys. Then you can save or load your map in various formats, currently supported are XML and Simple Trainer format (objects.ini).
The video uploaded to YouTube by GTA-Modding.com is from August and the mod has been updated since then, but it shows how you can fly around with freecam and spawn objects from a list. Go to 3:40 to watch the demonstrator dump a humpback whale or three across a road.
Head to moddb.com if you want to give it a try.
