Free_Electron gave you the exact answer. Not knowing your level of experience with Altium, I'll spell out the steps a bit more explicitly:
1. Use "Design->Board Shape->Create Primitives from Board Shape". Create the primitives in a mechanical layer, and make sure nothing else is in that layer.
2. You should be using an OutJob to create the Gerbers. Right click the "Gerbers" line, select config, go to the "layers" tab, and check the mechanical layer containing your outline.
3. Document your convention, including the name of the outline Gerber, in your README file.
Dave