Surprised there isn't a link provided within the sticky'd tutorial thread as I'm sure I've seen plenty of them but all you need to know is within:
http://www.graal.net/index.php/Creation/Dev/GMap
Though I suppose the one thing this lacks is a good example, you'll want to open notepad or wordpad and input something like this:
PHP Code:
GRMAP001
WIDTH 3
HEIGHT 3
LEVELNAMES
"yourlevela1.nw","yourlevela2.nw","yourlevela3.nw"
"yourlevelb1.nw","yourlevelb2.nw","yourlevelb3.nw"
"yourlevelc1.nw","yourlevelc2.nw","yourlevelc3.nw"
LEVELNAMESEND
Changing the width/height and level names to whatever is desired, click "Save As", and type "nameofyourmap.gmap" for it to save with the gmap extension.
When complete and uploaded you'll need to add the file into serveroptions, such as:
gmaps=nameofyourmap,nameofyourmap2,nameofyourmap3
You'll also need to ensure that the folder configuration allows for your gmap to be downloaded, such as by setting:
level gmaps/*.gmap
Getting the gmap working once everything is in place however seems to be a matter of luck, you'll likely have to do a "/updatelevel nameofyourgmap.gmap" within RC, sometimes it happens straight away other times it can take a random amount of time, as mentioned if the levels are already in use you'll need to restart the server.