force grayscale

[casueps]

The driver for my printer does not allow choosing between color or
grayscale, so I can only choose if the program options allow it. I want to make a filter in cups so that every user that prints through it with any program can print in grayscale.
I found the ghostscript command that reads a color PS file and outputs a grayscale PS file but I don't know how to insert it into the printer  options. Can you help me?