.Bm - bottom margin

This gives the number of blank lines to be placed in the bottom margin of the page.  These lines add lines to the page height beyond the number of lines specified in the .ps command.  This is set using the syntax

Localfileidentifier.bm=number;

or in the example putex5.gms

my1.bm=13;

This only works when the page control (pc) option is 0.