The gerber board edge layer is a set of lines that specify the outside edge of the PCB. The width of the line in the gerber is not important, the centre of the line becomes the outside edge of the PCB.

The router bit that cuts the edges of the board is 0.8mm wide, so this is the standard minimum width for any slots or detailed facets of routed edges.

Normally, the PCB will be made with no copper within approx 0.2mm of the board edge. This is "pullback" and it is done automatically by the PCB fab.

If you would like a PCB with copper all the way to the edge (for instance, for copper edge connectors or making a PCB-based SMD module) then ask for "no pullback" when placing your order.