[Tyhm]

In the extant level editor, if you have a properly set gmap and load any of the levels on it, then when you load a single level IN said editor and scroll to the side, you can see the adjacent levels. If you drag the four-point-arrow over the level-edge-warps, you can then edit the next level over. I believe if you save after having edited levels in this way it even saves all the levels you've been wrapping between, handy for when you put a tree right at the intersection of four levels.

Setting a GMAP properly though, that I no longer recall. I believe it's clientside setmap string,string,x,y;, but don't quote me on it.