I had a silver arowana that I adopted that I cut the gills on and they grew back fine. Took about 4-5 months for complete regrowth. Biggest thing is to get some fish anesthesia. I used some stuff called Finquel (MS-222) from argent labs. you can buy it online just do a search.
Mix the finqel according to the directions in a seperate container. Get a good pair of sharp scissors and boil them to sterilize. Get a clean pan large enough for your fish and place wet paper towels in it. Place you fish in the container with the finquel. Wait for him to turn upside down and his breathing to slow. Then place him into the pan with the moist papertowels. You need to keep him moist. Trim his gills you only have about 3 min before he starts to wake up if you druged him correctly. I took some antibiotics and rubbed them on the trimmed areas. Then place him back into his tank over of a air stone. He should come around in about 1 min. If this does not happen hold him by the tail and move him back and forth. Once good oxygenated water starts flowing over the gills the anesthesia wears off pretty quickly.