CopperCam is a very good choice.
It's wonderfully easy to use and is rather interesting to watch as it graphically processes the Gerber file.
I have been using it with Altium to produce tool paths for simulation purposes.
Once my CNC mill is completed, it will be making a few PCBs with it .
Please feel free to upload a small sample of your top copper layer, and I can run it thru Coppercamel for you.
I can then upload the .NC file, so you can try it out on your machine.