Gill cut is fine but you must humanely disconnect the control box first!!
Tap on head then immediate gill cut does two things.
1. Stuns the fish insensible with no chance of regaining consciousness. (animal welfare)
2. Prevents the build up of lactic acid in the flesh that would detract from eating quality and shelf life.
Both, therefore, ensuring the fish is maintained in the best possible condition.
Extensive research has been conducted with domestically produced table fish and the principals apply to all slaughtered fish where practical. This is why your own rod caught fish, taste that much better than those that have spent two hours at the back of a trawl net, under 3 tons of their mates.
T