GM1 is the outline of the board,and GKO is the routing keep out layer,someone used it to define the board outline.
If these two layers not covered the trace , via or pad of the other layers , then no problem. You just define layers, when you submit 'Gerber files' to manufacturer.
If you have special needs, you can add readme file submitted to the manufacturer. I do not know the other manufacturers, but pcbwing can support your requirements.