The GH1 play area is in East Asia. around what was once the East Sea. I think I said once that each world map tile was 10 or 15 km across; in the Ladon quest, the explorer says that Tokto is several hundred kilometers south of Namok, which makes that scale about right. The coastline is much different that it is in the 20th Century due to megaengineering projects undertaken by the superpowers.
The Joseon Green Zone is very large, covering much of Northern Asia. Joseon itself is in what was once Russia. You are right that Joseon is an old name for what is now Korea; the more common spelling over here is "Choseon", though Joseon is more accurate to pronunciation for English speakers. I chose Joseon because of a pun- Joseon = "Joe's Son" = something created by Joe. The FedTer does include more than one green zone, though I haven't worked out all the details yet.
In-game, it's named the Joseon Green Zone because Joseon was the first city to rise to prominence after the night of fire. Namok has argued that the Joseon Green Zone should be separate from the Namok Green Zone, which extends into the Pacific ocean and is separated from the Joseon Green Zone by a band of now-reclaimed dead zone running through Gyori.
Green zones are areas in the world that support abundant life. Dead zones are areas which don't. By scientific (in-universe) definition a healthy desert would be counted as a green zone, but by popular definition most people would regard it as a dead zone. Using modern technology it is possible to reclaim all but the most heavily polluted of the dead zones, though this is very expensive.
I believe Kist is within the fringes of the green zone, but it has not been incorporated into the FedTer. By the time of GH2 it has been incorporated, and Markheim Fortress is considering joining as well.
There are two kinds of dead-zone residents: ravagers and fortress-dwellers. In general, they hate one another. Fortress-dwellers live in large, armored enclaves. Typically they rely on ancient technology or local micro-greenzones for food, water, and other necessities. Ravagers are nomads. It's generally believed that they move into an area, use up all its resources, and then move on. While this is true of some tribes, it's not true of all. Some ravager groups are true nomads, returning to the same places over a cycle of many years. All the major cities of GH1 started out as fortresses.
