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I happen to have my 2005 F-350 King Ranch hooked up to my horse trailer right now.
For my integrated trailer brake controller, I can dial the gain back with the "minus" button all the way down to 0.0. I can then dial the gain up with the "plus" button from 0.0 up to 10. in .5 increments.
I've got a couple of gooseneck trailers and keep the brakes on them serviced regularly and the truck's trailer controller has always worked flawlessly for me. I can lock the trailer brakes up with the controller's manual "slider" engagement at will.
I would hook your truck up to another trailer with known good brakes and see what those results are prior to troubleshooting the controller.