Open a text document in notepad, ill give you an example of a gmap file.

GRMAP001
WIDTH 2
HEIGHT 1
LEVELNAMES
"levelname_a1.nw","levelname_b1.nw"
LEVELNAMESEND


Width = how many levels across the gmap is so as you can see its set as 2 so it will be 2 levels next to each other
Height = how many levels down the gmap is, its set to 1 so only one level down like a normal level.
levelnames are the names of the levels used for the gmap. so they start with the gmap name forexample the gmap file can be called era_themepark.gmap and the 2 levels will be name era_themepark_a1.nw and era_themepark_b1.nw
the letters are the columns and the numbers are the rows so since theres a and b like we set the width before, a comes first and then b will be to the right of it and they will both be on the row 1.
Once you create it save the file as a .gmap file

Probably badly explained and someone could do better but i just wanted to make a quick reply for you. You can also use the level generator that comes with the graal pc client, but i felt that was more awkward and annoying to use.