The KC60 Pro PCB can be flashed using TKG. The TKG Toolkit can be found HERE. You can follow the KEYCHATTER GUIDE to see how to use TKG on Windows or MY GUIDE to use it on a Mac, but there are some differences with this specific PCB that you'll need to make.
Here is a LINK to an example, standard layout for the KC60. If you'd like to change some of the keys to other functions or add more layers, I recommend looking at the tkg.io HELP PAGE to see how you should name the keys.
Once you have your layout complete, copy/paste the raw data from your first layer into tkg.io. Then do the same for your other layers. Your FN keys should show up, and you can assign them as you like. Note that LED control is a function option. In the standard layout I've linked above, FN1 is is intended to be assigned to toggle LED, FN2 is increase brightness, and FN3 is decrease.
Once you have downloaded your layout as an .eep file, run the TKG setup. Choose GH60 Rev. A/B/C. Then, flash the .eep file. You'll have to jump THESE TWO PADS to reflash. I've found that if you connect them, then hit yes ("Y") to confirm the reflash, and then remove the connection, it works the best.