Most non-laser printers suffer from the fact that their dots are rather large. This results in pictures coming out too dark. If you experience this problem you should use your own transfer function. Simply create the following file in the ghostscript lib-dir and add its name to the gs call just before the actual file. You may need to tweak the actual values to fit your printer.
Lower values result in a brighter print.
It is also possible to mend printers that have some kind of colour fault by tweaking these values.