Do you have the driver installed for usb to serial?
Whether it is gerber. Or not, it needs the driver or it becomes a "composite device" (nice word for nothing)
After the driver is loaded we need to match the settings from the adapter to the plotter
Remember, one side of the adapter/cable is plug and play (after the driver loads) but the plotter itself is dumb as a stump the settings (9600, none, one xon/xoff OR WHATEVER YOUR PLOTTER IS SET FOR. ) Have to match or nothing happens
Download the user manual, look at the dip switch and we can help if necessary, but first get that driver loaded for the usb