Edge plating is something you need to ask from your PCB manufacturer. What you need/might want to do in your PCB is just to add copper up to the edges of your PCB, where you want the copper plating to be:
So adding filled shape all along the edges of where you want the plating to be, you can then ask your manufacturer to make these edges plated.
As far as I know, in gerber the edge plating cannot be defined, as in gerber you export layer-by-layer data.
In eurocircuits for example you are given these options when uploading a PCB:
So in order to have copper up to the board edge you need to use filled shape, as the filled zones will keep their distance from the edge board (providing you have made your design rules to have a >0 distance from edges.
Also, make sure that your manufacturer understands where you want the plating(s) to be and where these plating are connected (if connected) on each layers of your pcb as its a tricky process that (almost always) is configured by hand from the manufacturer.