You have to unlock the .img with gimgunlock before splitting it with Gmaptool.
show me only a white square (i tried modifing zoom level and details but... nothing)
Zoom in.

From GMT readme:
Gmt create very simple basic map, just a minimum data to make Mapsource to see the map. This is OK for small area map but not good enough for big maps like City Navigators.You can improve preview map adding details form basemap. (How to instructions follow.)