The simplest way is to install a HP Postscript printer driver, eg HP Laserjet 5si PS. Set it up to print to file. When you print your gerber, the print driver will prompt you for a file name and generate a postscript output.
The problem with generating postscript by printing
on PS printer is that you cannot get
1:1 ratio. PS printer drivers decrease size
of the printout. Decrease is about 0,5 - 1%.
Please check it.