Hi Luciano,
It sounds like you want a layer that occupies all the space in your design not occupied by the current layers/cells in the design. I would do a NOT operation in this case ($L NOT). Here's an example script.
#create new layout
set ll [layout create]
#create new topcell
$ll create cell newCell
#create layers in cell
$ll create layer 3
$ll create layer 4
$ll create layer 50
#create some random shapes on 1 layer (3)
$ll create polygon newCell 3 10 10 100 100
$ll create polygon newCell 3 120 200 200 100
$ll create polygon newCell 3 140 50 300 10
#get the bounding box of the entire cell
set cellCoords [$ll bbox newCell]
#create a new layer the same size as the bounding box
eval $ll create polygon newCell 4 $cellCoords
#Perform a NOT between the random shapes and the blanket layer to get the difference layer on 50
$ll NOT 4 3 50
#create the GDS
$ll gdsout output.gds
matt