theres a couple ways you can do it.
You can use a gmap generator and all you do is type in the width/length of the gmap you want and then the gmap name. (Unfortunately i always lose the link to this generator)
Another way is to use graal's generator tool.
And you can do it manually.
Let me see if i can find that link, it will probablly be the easiest for you
Edit: heres the link for the gmap generator.
http://www.graal.us/tools/generatelevels.html
Crow and a couple others have made their own, but i prefer using the link atm.