Homebrew PCBs

I need a little help to transform my PCB milling machine in an UV 
laser PCB exposure one.
I've reverse engineered the Z, X and Y signals to obtain the laser 
on/off signals.
Now I need a program to convert the universal Gerber output to HPGL 
like for a pen plotter.
My laser head will move line by line.

Knows someone such a program, preferable free, please?

Cristian