I am not sure if this can be done in Altium, but you can simply invert gerber files.
- In case you are sending your PCB to a PCB manufacturer, just make a note: Negative / Positive.
- If you would like to print the layer e.g. if you are making the PCB at home, you can use for example the free version of ViewMate (
http://www.pentalogix.com/viewmate.php ), invert the layers there and print them out.
I attached also examples of the layer printed into PDF.