Actually, I thought there were a number of tools from the g-code side of the world for converting either gerber or HPGL to G-code, and most 'serious' PCB editors can export to gerber and/or HPGL. I think that means you're back to asking for general PCB package recommendations. There is kicad and gEDA (gnu SCH and PCB) for free; I'm pretty convinced that "simple" is pretty hard to come by in this sort of app...