Using GerbView to combine positive and negative layers into a single Gerber file.

Some PCB layout programs generates traces as a positive Gerber file, and power/ground plane as a negative Gerber file, which is supposed to be combined to create a finished Gerber file.
You can use GerbView to perform this task.
To do this you can do the following:

1. Load the negative power/ground plane file. 
2. On the right side of the screen you will see a list of available settings for the loaded Gerber file. In the list you will find a setting  named "invert". Enable this option to create a positive display of the layer.

Sample screenshot after adding layer and inverting this:

Then load the second layer and move it to the top of the layer list.

You should now see something like this:

Finally, save the two layers to a single Gerber file using Save As option:

Press OK and select a file name for the new Gerber file.

Final result shown is below:

GerbView® is a Software Companions product, and you will find more information about it here:

http://www.gerbview.com/
and here:
http://www.softwarecompanions.com/gerbview.html