well I see that some things are not stated there but:
In the new nextmap.gsl you have to include a load world function, here is example from athos:
void MainGame()
{
//load the world file:
NewWorld();
//begin loading the world
StartLoadingWorld("../worlds/level1.wld");
while(!DoneLoadingWorld())
Delay(0.01f);
GameManager@ game = GetGameManager();
Player@ player = game.CreatePlayer("Player",true);
Object@ playerObject = game.GetWorld().GetObject("playerDummy");
Viewport@ viewport = game.GetViewport("Main");
player.AddToInventory(playerObject);
//fade out to black
viewport.FadeOut(0);
SetRelativeMouseMode(true);
So as it says in the text:
This will call the game script for your next level, which activates the loading of that level etc as described in the docs.
Pretty simple, no?
So the nextmap.gls will be loaded that actives the StartLoadingWorld("../worlds/level1.wld");
Hope this helps