Heightmap resolution is way too low for a map with the detail level *and* size of GTA 5's. It could possibly be done with meshes, but they tend to lag more so that would be a poor idea.
According to a quick internet search, the GTA V map size is 49km[SUP]2[/SUP], which is 7 kilometers on one side. 9km[SUP]2[/SUP] is only 3 kilometers on one side. That's over four times the area of the map you mention, and most maps simply aren't even that big. And while, yes, you can make maps larger still, it comes at the cost of resolution. Unless an 8192x8192 heightmap can be generated....
There's already a conversion of the airport from GTA IV, and that was pushing BeamNG's capabilities, my GTX 970 couldn't hold a framerate
Why use a 8192x8192 heightmap when you can just use normal sized ones and stick them all together. (This would be a great feature for BeamNG.)
I'm pretty sure you can have multiple terrrains, but the second and third and so on won't have collision. But I don't know why you'd want multiple normal size ones when you can have one big one. It doesn't change anything.