In the PCB editor, hit 'L' to bring up the layers menu, then select "Enable" and "Show" next to the mechanical layer of your choice. Typically Mechanical Layer 1 is used for the board outline. Draw the board outline on that layer, then select the outline objects and hit D,S,D to use them to define the board outline.
In your gerber outjob, make sure that your outline layer is selected for plotting.